text/template: fix newline counting in raw strings

lexRawQuote already uses the next method, which keeps track of newlines
on a character by character basis. Adding up newlines in emit again
results in the newlines being counted twice, which can mean bad position
information in error messages.

Fix that, and add a test.

Fixes #27319.

Change-Id: Id803be065c541412dc808d388bc6d8a86a0de41e
Reviewed-on: https://go-review.googlesource.com/131996


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>