feat(comment): allow commentstring to be determined from node metadata

**Problem:** Some weird languages have different comment syntax
depending on the location in the code, and we do not have a way to
determine the correct `commentstring` for these special cases.

**Solution:** Allow queries to specify `commentstring` values in
metadata, allowing users/`nvim-treesitter` to provide a better
commenting experience without hugely increasing the scope of the code in
core.

runtime/lua/vim/treesitter/query.lua

@@ -608,6 +608,7 @@ predicate_handlers['any-vim-match?'] = predicate_handlers['any-match?']
 ---@class vim.treesitter.query.TSMetadata
 ---@field range? Range
 ---@field conceal? string
+---@field bo.commentstring? string
 ---@field [integer]? vim.treesitter.query.TSMetadata
 ---@field [string]? integer|string