[GO-2023-1703] Backticks not treated as string delimiters in html/template

Templates do not properly consider backticks (`) as Javascript string
delimiters, and do not escape them as expected.

Backticks are used, since ES6, for JS template literals. If a template contains
a Go template action within a Javascript template literal, the contents of the
action can be used to terminate the literal, injecting arbitrary Javascript code
into the Go template.

As ES6 template literals are rather complex, and themselves can do string
interpolation, the decision was made to simply disallow Go template actions from
being used inside of them (e.g. "var a = {{.}}"), since there is no obviously
safe way to allow this behavior. This takes the same approach as
github.com/google/safehtml.

With fix, Template.Parse returns an Error when it encounters templates like
this, with an ErrorCode of value 12. This ErrorCode is currently unexported, but
will be exported in the release of Go 1.21.

Users who rely on the previous behavior can re-enable it using the GODEBUG flag
jstmpllitinterp=1, with the caveat that backticks will now be escaped. This
should be used with caution.