text/scanner: don't allow Float exponents with no mantissa

Previously Scanner would allow float literals like "1.5e" and "1e+"
that weren't actually valid Go float literals, and also not valid
when passed to ParseFloat. This commit fixes that behaviour to match
the documentation ("recognizes all literals as defined by the Go
language specification"), and Scanner emits an error in these cases.

Fixes #26374

Change-Id: I6855402ea43febb448c6dff105b9578e31803c01
Reviewed-on: https://go-review.googlesource.com/129095


Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>