skip `del` elements when matching productions

[bakkot]

https://github.com/tc39/ecmarkup/pull/449 makes the logic for matching productions at least work in the presence of <ins> and <del>, but really you don't want to include <del>'d things in the matching logic. For example, if you define Foo : Bar, then Foo : Bar <del>Baz</del> should match that. Conversely, if you define Foo: Bar Baz, then Foo : Bar <del>Baz</del> should not match that.

The matching logic should skip anything within a <del> tag.