plugins/telescope: support non-builtin keymaps

[MattSturgeon]

Use the :Telescope command instead of relying directly on
require("telescope.builtin").

To quote upstream docs:

All telescope.nvim functions are wrapped in vim commands for easy access, tab completions and setting options.

Fixes #1493

[MattSturgeon]

<cmd> mappings are not echod, so the keymapsSilent option now has no effect and could be deprecated.

[GaetanLepage]

<cmd> mappings are not echod, so the keymapsSilent option now has no effect and could be deprecated.

Let's do that in this PR then.

[MattSturgeon]

I think there's a design decision to make here.

Using using the :Telescope command is simpler, since both builtin and extension pickers are directly available, and options can also be passed to the picker as part of the command.

The viml api can be called using commands like: :Telescope find_files hidden=true layout_config={"prompt_position":"top"}

This involves setting options using an = and using viml syntax for lists and dictionaries when the corresponding lua function requires a table.

However :h telescope.command does point out some limitations:

The viml api is more indirect, as first the command must be parsed to the
relevant lua equivalent, which brings some limitations.

One limitation of the viml api is that there can be no spaces in any of the
options.

For example, if you want to use the cwd option for find_files to
specify that you only want to search within the folder /foo bar/subfolder/
you could not do that using the viml api, as the path name contains a space.

It's also unclear whether all third-party extensions will expose their picker functions via the :Telescope command...

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That said, this is already more powerful than the current approach, since the current telescope keymap wrapper doesn't allow passing any option arguments to the picker function anyway. Nor does it allow using third-party (extension) pickers.

If we decide we don't like the :Telescope approach, the only other way I can think of to (directly) support third-party picker functions is by adding an option along the lines of keymap.extension = mkNullOrOption str "If action is a third-party extension function, this should be the name of the extension.". I think this is more cumbersome from a UX perspective, though.

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We could also augment either the old or new system by allowing action to be a raw string. This could be used as the body of a lua function where we've already defined some local variables (e.g. telescope, builtins, extensions). This way the user can use the lua API to call telescope functions.

[MattSturgeon]

@GaetanLepage could you re-review when you have chance?

[GaetanLepage]

Nice ! LGTM.