feat: specify icons per buffer activity/state

[Iron-E]

This PR allows customization of the bufferline.icons setting per activity / state, like so:

require'bufferline'.setup {
  icons = {
    -- Configure the base icons on the bufferline.
    buffer_index = false,
    buffer_number = false,
    close = '',
    diagnostics = {…}, -- moved here from the `diagnostics` option
    filetype = {
      -- Sets the icon's highlight group.
      -- If false, will use nvim-web-devicons colors
      custom_colors = false,

      -- Requires `nvim-web-devicons` if `true`
      enabled = true,
    },
    inactive = {separator = {left = '▎'}},
    separator = {left = '▎'},

    -- Configure the icons on the bufferline when modified or pinned.
    -- Supports all the base icon options.
    modified = {button = '●'},
    pinned = {button = '車'},

    -- Configure the icons on the bufferline based on the visibility of a buffer.
    -- Supports all the base icon options, plus `modified` and `pinned`.
    alternate = {filetype = {enabled = false}},
    current = {buffer_index = true},
    inactive = {button = '×'},
    visible = {modified = {buffer_number = false}},
  },
}

---

When multiple options match (e.g. icons.separator, icons.modified.separator, and icons.alternate.separator), precedence works as follows:

1. icons.(alternate|current|inactive|visible).(modified|pinned).foo, if the buffer is modified or pinned
2. icons.(modified|pinned).foo, if the buffer is modified or pinned
3. icons.(alternate|current|inactive|visible).foo
4. icons.foo

Users who specify their icons/icon_separator_active/etc options the old way are prompted with a deprecation notice, but no action is necessary. The old options are automatically translated to the new format, and this will continue until a theoretical v2.0.0 releases.

---

This seemed like the best compromise between letting a user provide a function (my initial proposal) and adhering to the simplicity of user configuration (romgrk's preference).

---

Closes #46.
Closes #243.
Closes #379.

[otavioschwanck]

Extremely WIP.

This PR allows customization of the bufferline.icons setting per activity / state, like so:

--- @class bufferline.options.icons.buffer
--- @field buffer_number? boolean iff `true`, show the `bufnr` for the associated buffer.
--- @field buffer_index? boolean iff `true`, show the index of the associated buffer with respect to the ordering of the buffers in the tabline.
--- @field file_icon? boolean iff `true`, show the `devicons` for the associated buffer's `filetype`.

--- @class bufferline.options.icons.state: bufferline.options.icons.buffer
--- @field modified? bufferline.options.icons.buffer the icons used for an modified buffer
--- @field pinned? bufferline.options.icons.buffer the icons used for a pinned buffer

--- @class bufferline.options.icons
--- @field alternate? bufferline.options.icons.state the icons used for an alternate buffer
--- @field current? bufferline.options.icons.state the icons for the current buffer
--- @field default? bufferline.options.icons.state the icons used when no other filter matches
--- @field inactive? bufferline.options.icons.state the icons for inactive buffers
--- @field visible? bufferline.options.icons.state the icons for visible buffers

When multiple definitions match, they most specific definition applies on top of its defaults. For example:

require'bufferline'.setup {
  icons = {
    default = {buffer_number = true, pinned = {buffer_index = true}},
    visible = {file_icon = true, modified = {file_icon = false}},
  },
}

• A visible buffer will show its file icon.
• A visible, pinned buffer will show its file icon and buffer index.
• A visible, modified buffer will show no icons (since visible.modified.file_icon overrides visible.file_icon).
• A visible, modified, and pinned buffer will show only the buffer index.
• A non-visible, pinned buffer will show its buffer index and buffer number.
• A non-visible, non-pinned buffer will show its buffer number.

There is automatic translation done for users who specify their options in the "old" way, i.e.:

--- @alias bufferline.options.icons.preset boolean|"both"|"buffer_number_with_icon"|"buffer_numbers"|"numbers"

--- @type {[bufferline.options.icons.preset]: bufferline.options.icons}
local PRESETS = {
  [false] = {},
  [true] = {default = {file_icon = true}},
  both = {default = {buffer_index = true, file_icon = true}},
  buffer_number_with_icon = {default = {buffer_number = true, file_icon = true}},
  buffer_numbers = {default = {buffer_number = true}},
  numbers = {default = {buffer_index = true}},
}

This seemed like the best compromise between letting a user provide a function (my initial proposal) and adhering to the simplicity of user configuration (romgrk's preference).

Closes #379.

Very nice, a nice think to have maybe is the pinned tabs be always visible (for a next task)

[Iron-E]

@otavioschwanck feel free to try this PR now, I've been using it for about a day and haven't gotten any errors (but that usually means they're hiding, so an extra tester would be helpful!). I believe your desired configuration would be:

require'bufferline'.setup {
  icons = {
    pinned = {
      buffer_index = true,
    },
  },
}

---

Feel free to make another issue for keeping pinned tabs visible.

[otavioschwanck]

Some issues:

If we set the deprecated options, barbar throw an error and close itself.

i think the close = 'icon' is a strange option.

i had to do that:

        icons = {
          pinned = { buffer_index = true, close = '󰐃' },
        },

Maybe something like: left, icon or button, idk,

[Iron-E]

Thanks for sacrificing some time out of your busy schedule! I appreciate it. This is a lot of code; I wanted to get your OK on the high-level concepts even if you didn't have time to go line-by-line.

I'll test it one more time before merging.