feat(comment): add built-in commenting

Design

- Enable commenting support only through `gc` mappings for simplicity.
  No ability to configure, no Lua module, no user commands. Yet.

- Overall implementation is a simplified version of 'mini.comment'
  module of 'echasnovski/mini.nvim' adapted to be a better suit for
  core. It basically means reducing code paths which use only specific
  fixed set of plugin config.

  All used options are default except `pad_comment_parts = false`. This
  means that 'commentstring' option is used as is without forcing single
  space inner padding.

As 'tpope/vim-commentary' was considered for inclusion earlier, here is
a quick summary of how this commit differs from it:

- **User-facing features**. Both implement similar user-facing mappings.
  This commit does not include `gcu` which is essentially a `gcgc`.
  There are no commands, events, or configuration in this commit.

- **Size**. Both have reasonably comparable number of lines of code,
  while this commit has more comments in tricky areas.

- **Maintainability**. This commit has (purely subjectively) better
  readability, tests, and Lua types.

- **Configurability**. This commit has no user configuration, while
  'vim-commentary' has some (partially as a counter-measure to possibly
  modifying 'commentstring' option).

- **Extra features**:
    - This commit supports tree-sitter by computing `'commentstring'`
      option under cursor, which can matter in presence of tree-sitter
      injected languages.

    - This commit comments blank lines while 'tpope/vim-commentary' does
      not. At the same time, blank lines are not taken into account when
      deciding the toggle action.

    - This commit has much better speed on larger chunks of lines (like
      above 1000). This is thanks to using `nvim_buf_set_lines()` to set
      all new lines at once, and not with `vim.fn.setline()`.

runtime/lua/vim/_defaults.lua

@@ -114,6 +114,24 @@ do
       do_open(table.concat(vim.iter(lines):map(vim.trim):totable()))
     end, { desc = gx_desc })
   end
+
+  --- Default maps for built-in commenting
+  do
+    local operator_rhs = function()
+      return require('vim._comment').operator()
+    end
+    vim.keymap.set({ 'n', 'x' }, 'gc', operator_rhs, { expr = true, desc = 'Toggle comment' })
+
+    local line_rhs = function()
+      return require('vim._comment').operator() .. '_'
+    end
+    vim.keymap.set('n', 'gcc', line_rhs, { expr = true, desc = 'Toggle comment line' })
+
+    local textobject_rhs = function()
+      require('vim._comment').textobject()
+    end
+    vim.keymap.set({ 'o' }, 'gc', textobject_rhs, { desc = 'Comment textobject' })
+  end
 end
 
 --- Default menus