Move template processing out of experimental and enable it. Users
can set the option 'templateSupport' to false to disable it.
Also, add a new flag 'templateExtensions', defaulting to ['tmpl','gotmpl']
to let the user provide a list of extensions of files
to be considered as template files.
Change-Id: I724387738c1632256999cda304d6cf9fa48ed91d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/362241
Run-TryBot: Peter Weinberger <pjw@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Peter Weinberger <pjw@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>