html/template: attach functions to namespace

html/template: attach functions to namespace

The text/template functions are stored in a data structure shared by
all related templates, so do the same with the original, unwrapped,
functions on the html/template side.

For #39807
Fixes #43295

Change-Id: I9f64a0a601f1151c863a2833b5be2baf649b6cef
Reviewed-on: https://go-review.googlesource.com/c/go/+/279492
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>