Support wildcard nodes `_`

[stsewd]

I also gave the anonymous nodes a field name, otherwise we aren't able to capture the name, I'll update the playground and our queries.

Wasn't sure if I should give the wildcard node a name or leave it anonymous, suggestions are welcome!

Ref:

• https://github.com/nvim-treesitter/nvim-treesitter/issues/1405#issuecomment-877723263
• https://github.com/tree-sitter/tree-sitter/issues/1253