feat: allow enabling extensions per buftype

[martinparadiso]

Context and motivation

I was writing an extension for the special terminal buffer
and realized in has no filetype like the quickfix window,
instead filetype is empty and the buftype is 'terminal'.
Currently here is no direct way to set an special bufferline
for the terminal buffer.

Proposed change

Add a buftypes table to the extension table, allowing to
filter extensions by buftype; and the corresponding
implementation.

Some basic test cases were added.

Example extension

Example of an extension for terminal buffers:

Source code

local terminal_extension = {
    sections = {

        lualine_a = { function()
            return 'TERMINAL'
        end },

        lualine_b = {
            function()
                local name = vim.api.nvim_buf_get_name(0)
                local match = string.match(vim.split(name, ' ')[1], 'term:.*:(%a+)')
                return match ~= nil and match or vim.fn.fnamemodify(vim.env.SHELL, ':t')
            end },
        lualine_c = {
            function()
                return vim.b.term_title
            end
        },
        lualine_x = {},
        lualine_y = {

            function()
                local dev, _ = require('nvim-web-devicons').get_icon('zsh')
                return dev
            end
        },
        lualine_z = {},
    },
    buftypes = { 'terminal' },
}

[shadmansaleh]

Can you include the terminal extension too? It seems nice to have plus will be an example implementation.

[martinparadiso]

Done, should be working with and without web-devicons.