Which face to use for highlighting interpolation delimiters?

[Dmitry Gutov]

E.g. ${ and } in "String literal with #{var} in the middle".

So far it seems that the ts modes haven't managed to agree on one yet.

js-ts-mode uses font-lock-delimiter-face, and
typescript-ts-mode uses font-lock-builtin-face (for template_substitution).

Maybe we need a new one.

While we're at it, maybe a face for regexps literals as well?