html/template: add support for JavaScript modules

html/template does not properly treat JavaScript code as
JavaScript when using a <script> tag with "module" set as
the type attribute.

See also:
https://www.w3.org/TR/html5/semantics-scripting.html#element-attrdef-script-type and
https://html.spec.whatwg.org/multipage/scripting.html#the-script-element:module-script-2

Original change from tomut at https://golang.org/cl/135417

Fixes #31327

Change-Id: I6239be69cd7994990d091400664e4474124a98fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/175218


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>