Add support for string unescape

[steve-ky]

Checklist

• [x] I have read the contribution guidelines.
• [x] make test passes.
• [x] make lint passes.
• [x] New and changed code is covered by tests.
• [ ] Performance improvements include benchmarks.
• [x] Changes match the documented (not just the implemented) behavior of Shopify.

Support double quoted string unescape, single quoted string won't unescape.

[danog]

Mind rebasing pls?