Add initial variable processing

[domenic]

This supports the work in whatwg/html#11392 with a new processor found in variables.rs, documented there.

This required updating the parser to store line numbers for each element, which changed a lot of test call sites.

@jmdyck you may not want to review all the code, but the tests at the bottom of variables.rs might be worthwhile checking over.

[domenic]

I guess there will be a chicken-egg problem with whether whatwg/html#11392 or this lands first, since html-build will refuse to publish unless the "Run against whatwg/html" succeeds, and html will not let us merge a PR which fails validation.

I suppose the best solution would be to land the attribute stripping/conversion functionality first, then add the variable checking afterward.

I'll probably end up doing a simpler version of that by commenting out some parts of the error-on-orphaned-vars code, landing this, landing whatwg/html#11392, then landing a second PR that uncomments that check.