text/template: add a slice function to the predefined global functions

text/template: add a slice function to the predefined global functions

The new slice function returns the result of slicing its first argument by
the following arguments. Thus {{slice x 1 3}} is, in Go syntax, x[1:3].
Each sliced item must be a string, slice, or array.

Closed #30153

RELNOTE=yes

Change-Id: I63188c422848cee3d383a64dc4d046e3a1767c63
Reviewed-on: https://go-review.googlesource.com/c/go/+/161762
Reviewed-by: Rob Pike <r@golang.org>