refactor(parser): Inverse stacks, simplify

[fb55]

There is now an internal htmlMode property that caches the use of !this.options.xmlMode all across the file. Also foreignContext is now reset, and will always contain an element.

[coveralls]

Pull Request Test Coverage Report for Build 4934815445

• 45 of 45 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0006%) to 99.654%

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Totals
Change from base Build 4934434651:	0.0006%
Covered Lines:	1730
Relevant Lines:	1736

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💛 - Coveralls