html/template: support parsing complex JS template literals
This change undoes the restrictions added in CL 482079, which added a
blanket ban on using actions within JS template literal strings, and
adds logic to support actions while properly applies contextual escaping
based on the correct context within the literal.
Since template literals can contain both normal strings, and nested JS
contexts, logic is required to properly track those context switches
during parsing.
ErrJsTmplLit is deprecated, and the GODEBUG flag jstmpllitinterp no
longer does anything.
Fixes #61619
Change-Id: I0338cc6f663723267b8f7aaacc55aa28f60906f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/507995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org> Reviewed-by: Damien Neil <dneil@google.com>