Add support for string interpolation

[msteinbeck]

Thanks for providing this small, yet useful Emacs mode.

I noticed that double-quoted strings are highlighted properly, but single-quoted strings have no highlighting at all. I have two requests:

1. Also highlight single-quoted strings.
2. Emphasize variables and expressions inside single-quoted strings.

[jcs090218]

I recommend you use TreeSitter base solution, like tree-sitter.el or treesit.el.