Need to unescape escape sequences before passing text to embedded formatter

[kt3k]

An example:

css`
.bg-\\[\\#86efac\\] {
  background-color: #86efac;
}
`

This currently causes syntax error with embedded language formatter (malva) because .bg-\\[\\#86efac\\] is invalid CSS selector. We need to unescape .bg-\\[\\#86efac\\] part before passing it to embedded formatter, and then escape it again.

ref https://github.com/denoland/fresh/blob/3900aa70d4b305080f42e5814bc832b22aee2d39/init/src/init.ts#L317-L319