Add integer value extended attribute support to the IDLParser

Integer values are described as valid extended attribute values in the
WebIDL spec: https://webidl.spec.whatwg.org/#idl-extended-attributes
This CL adds support for them to the IDLParser at //tools/idl_parser/
along with an associated test.

This being added to enable the conversion of some Extensions API schema
files (which use integer value extended attributes) from an older
outdated IDL format to use modern WebIDL.

Fixed: 487349972
Change-Id: Ib0d0a0d76c8ca016ac082b36b0df0787bd2f350f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7604485
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Tim <tjudkins@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1590310}
NOKEYCHECK=True
GitOrigin-RevId: 3bee28813694258b967536c273822475bd8b3980