[harmony] Bug 2059621: Add simpler method to check if an extension is present

[CyberShadow]

Resurrection of #35.

Applied the suggestions by @dylanwh .

[mrenvoize]

Should we update the commit message of that first commit to reference the new bug number we have?

[mrenvoize]

QA review of this patchset (functional testing in Docker, live smoke test via browser, full sanity suite, perlcritic/perltidy) found no regressions or security concerns. has_extension() matches the semantics of the old grep-based check, and the recursion guard doesn't misfire against any current extension code (none override enabled() or query the extension list at load time).

One minor style nit: the new line in votes.cgi exceeded the project's 80-char limit and had inconsistent brace spacing per perltidy --profile=.perltidyrc. Non-blocking, but flagging as a follow-up:

-Bugzilla->has_extension('Voting') || ThrowCodeError('extension_disabled', { name => 'Voting' });
+Bugzilla->has_extension('Voting')
+  || ThrowCodeError('extension_disabled', {name => 'Voting'});

[mrenvoize]

All looking good here. :)