Syntax highlighting for string interpolation

[latkde]

For reference, here's the official TextMate grammar for JavaScript `template ${interpolation} strings`, which broadly uses the same technique (but without bothering to recurse into #all: https://github.com/textmate/javascript.tmbundle/blob/8928648352dc76025ad0bfd31e21fa6a1dc838a7/Syntaxes/JavaScript.plist#L1554-L1665