text/template: add "else with" action

text/template: add "else with" action

Add "else with" action will reduce the template complexity in some use cases(#57646).
This action will be added:
  {{with pipeline}} T1 {{else with pipeline}} T0 {{end}}.

Fixes #57646

Change-Id: I90ed546ab671805f753343b00bd3c9d1a1d5581d
Reviewed-on: https://go-review.googlesource.com/c/go/+/545376
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>