The unicode property escape syntax restricts unicode property names and
unicode property values to consist only of characters taken from the
[a-zA-Z0-9_] character class. See the spec at:

https://tc39.github.io/proposal-regexp-unicode-property-escapes/

In most cases, we do not actually need to validate that this is the
case, since subsequent property lookup in ICU will fail (and throw a
SyntaxError) if the given property does not exist.

However, there one special case. The ICU lookup takes the property name
as a null-terminated string, so it will accept carefully malformed
property names (e.g. '\p{Number\0[}'). This can end up confusing the
regexp parser.

With this CL, we explicitly restrict potential property names / values
to the character set as specified.

Bug: v8:4743, chromium:793793
Change-Id: Ic97deea8602571ec6793b79c4bb858e1c7597405
Reviewed-on: https://chromium-review.googlesource.com/824272Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50130}