fix(formatexpr)!: do not fallback to the built-in formatter

[wookayin]

This makes the behavior of formatexpr more consistent with
vim.lsp.formatexpr; do not run vim's builtin formatter.

Problem: When conform's formatexpr is used and (range or buffer) but
conform can't do formatting and there is no LSP server with the
formatting capabilities, it will fall back to the (wrong) built-in
formatting, which might result in simply concatenating all the words.

This is a breaking change (as a part of v5.0 with breaking change),
reverting the behavior introduced in 278bcd8 (#55). /cc @cryptomilk

[wookayin]

If one still would like to fallback to the built-in formatter, they can write a custom formatexpr function like (although not recommended):

function _G.formatexpr()
    if #require('conform').list_formatters_for_buffer() > 0 or
       #require("conform.lsp_format").get_format_clients() > 0
        return ('conform').formatexpr()  -- uses conform or LSP
    end
    return 1  -- fallback to the built-in formatexpr, see :help formatoptions
end

But ideally formatexpr should be set locally per buffer using LspAttach, FileType autocmds, rather than globally. Example:

  vim.api.nvim_create_autocmd('FileType', {
    pattern = vim.tbl_keys(require("conform").formatters_by_ft),
    group = vim.api.nvim_create_augroup('conform_formatexpr', { clear = true }),
    callback = function()
      vim.opt_local.formatexpr = 'v:lua.require("conform").formatexpr()'
    end,
  })

[wookayin]

@stevearc Any comments? I wish this were included in v5.0 together as we bump up the major version, because it's a breaking change.

[stevearc]

I think you make a good point: gw is always available for people that want the default behavior, and not falling back to the default formatexpr makes the behavior more consistent with the LSP formatexpr, which I generally support. The more conform's API behaves like LSP, the fewer surprises users will encounter.

I would argue that this isn't really a breaking change, it's just a behavior change. Nobody's config is going to throw errors because of this.

[wookayin]

Thank you for merging this!