Background: In clojure-ts-mode I've been capturing docstrings and
applying some limited syntax highlighting using an embedded markdown
parser. I'm only able to capture the full string, "quotes included". I
would like to be able to easily adjust the ranges captured to only
include the contents of the string, delimiters excluded. I have a
similar desire to capture the contents of a regular expression literal
and apply a nested regex grammar.

I've seen an offset mechanism used by the neovim tree-sitter integration
for similar purposes.

I believe the javascript/typescript modes could take advantage of this
with template strings. I've included a small test in the patch that
demonstrates this.