[WIP] nushell grammar for tree-sitter

1. unquoted strings

2. blocks vs records since we cannot backtrack, it is difficult to tell apart blocks and records. so currently, blocks are not parsed as values

3. cellpaths with numbers

4. command arguments are parsed as strings

here 2, -f, arg will all be parsed as strings

• for neovim users, please have a look at installation/neovim.md