Forbid long variable names that start with _

[ShabbyX]

Fixes #3783

[kenrussell]

Thanks for revising this test. A few pieces of feedback:

• Please make the change against the top-of-tree conformance tests which are under sdk/tests/ rather than the older conformance suite snapshots under conformance-suites/. We can backport the change after it lands, but tend to keep the older snapshots running under both the old and the new behavior.
• To make git logs easier to read and simplify rolls of the WebGL conformance suite into Chromium, please include details from the issue in the pull request's first message, and mention the Chromium issue number, even if it isn't publicly visible yet.
• Please add a new positive test like shader-with-1022-character-identifier.frag.html‎. Add it to 00_test_list.txt with --min-version=2.0.1. Recent PRs where you can follow the pattern: #3785 and #3782 .

If any of this is too tedious let me know and I can help prepare the PR.

[kenrussell]

Could you please also modify specs/latest/2.0/index.html, https://registry.khronos.org/webgl/specs/latest/2.0/#5.3.1 , revising it to the new behavior? That way the spec and tests are in sync.