text/template/parse: add a mode to skip func-check on parsing

Following the discussion on #34652 and the proposal of #36911 (gopls),
this CL adds an option to skip the function declartion check on parsing,
in order to make it possible to parse arbitrary template text files and
get their AST.

Fixed #38627

Change-Id: Id1e0360fc726b49dcdd49716ce25563ebaae6c10
Reviewed-on: https://go-review.googlesource.com/c/go/+/301493


Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>