text/template/parse: give the lexer more lookahead

[robpike]

https://go-review.googlesource.com/c/go/+/398475 and its issue #52165 are triggered by an inability to do sufficient lookahead, but that should be easy to do(but isn't) as the lexer operates on the full string. The submitted fix is clumsy but should be as easy as a call to strings.HasPrefix, and I want to make it that easy.

The lexer in robpike.io/ivy originated with this code but has been updated to avoid this problem. Do the same to text/template/parse. The required changes are modest and will not affect the public interface.

[gopherbot]

Change https://go.dev/cl/406476 mentions this issue: text/template/parse: make atTerminator more efficient