Enforce that soong config variables are identifiers

I want to be able to use certain characters to separate a namespace
from a variable, but currently it's possible for soong config namespaces
and variables to have any character in them.

Restrict them to just identifiers, which should cover all of our
existing usages.

Bug: 323382414
Test: Presubmits
Change-Id: I6d55dd3378ac4d257efde450189b81a4aa3932c3