text/template: allow using -}} with many spaces

lexSpace consumed all spaces, even if the last one was part of a right
delimiter like " -}}". Thus, "3  -}}" wouldn't lex as "3" and a right
delimiter, but as "3", "-", and "}}".

To fix that, make lexSpace stop if it encounters a right delimiter.

Fixes #30948.

Change-Id: I80a5546e5809e54f6823e2bf3a57a7e8808329be
Reviewed-on: https://go-review.googlesource.com/c/go/+/168457


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>