feat: raw strings: distinguish delimiters from actual string (#114)

We could set the delimiters to a different highlight group. Example
`@punctuation.special`:

![image](https://github.com/user-attachments/assets/4903f641-48fe-42f4-87dd-6245c644447d)

To see all available groups, hit `:h treesitter-highlight-groups`. I
could see:
- `@punctuation.special`
- `@punctuation.delimiter`
- `@tag.delimiter`
- `@string.escape`
- `@string.special`
- `@string.special.symbol`

Any opinions?

Edit: with highlights now in `nvim-treesitter`, this feels a bit weird.
But we can still maintain our own highlights :D

queries/nu/highlights.scm

@@ -90,6 +90,10 @@ file_path: (val_string) @variable.parameter
 (escaped_interpolated_content) @string
 (expr_interpolated ["(" ")"] @variable.parameter)
 
+(raw_string_begin) @punctuation.special
+(raw_string_content) @string
+(raw_string_end) @punctuation.special
+
 ;;; ---
 ;;; operators
 (expr_binary [