text/template: simplify line tracking in the lexer

First, move the strings.Count logic out of emit, since only itemText
requires that. Use it in those call sites. itemLeftDelim and
itemRightDelim cannot contain newlines, as they're the "{{" and "}}"
tokens.

Secondly, introduce a startLine lexer field so that we don't have to
keep track of it elsewhere. That's also a requirement to move the
strings.Count out of emit, as emit modifies the start position field.

Change-Id: I69175f403487607a8e5b561b3f1916ee9dc3c0c6
Reviewed-on: https://go-review.googlesource.com/132275


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>