Setup AST module inside syntax crate

[doriath]

Similar to rust-analyzer, we should create ast module inside syntax crate, that will contain the concrete AST types instead of CST (GreenNode)

[Freed-Wu]

So a plan of this project's is to create a vim parser like https://github.com/iamcco/vim-language-server/ 's https://github.com/vim-jp/vim-vimlparser .