Adds 'templateExtensions' (with default ["tmpl", "gotmpl"]) to control
which files gopls considers template files.
Adds template support documentation to features.md.
Fixesgolang/go#36911
Change-Id: If0920912bf3748d1c231b3b29e7a008da186bede
Reviewed-on: https://go-review.googlesource.com/c/tools/+/363360
Run-TryBot: Peter Weinberger <pjw@google.com>
Trust: Peter Weinberger <pjw@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>