Add validation for keywords related to numbers and strings.
It may be surprising that keywords like "minimum" or "minLength"
don't imply that the instance is a number or string.
But that is how JSON Schema is designed.
Change-Id: I8805a5fe91ee345931e1c989f174ea2d0be6665a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/667975
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
