Remove lookarounds from autolink extension patterns

[chenzhiguang]

Fix: https://github.com/dart-lang/markdown/issues/515

[coveralls]

Pull Request Test Coverage Report for Build 4186806802

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 96.064%

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Totals
Change from base Build 4179377546:	0.02%
Covered Lines:	1391
Relevant Lines:	1448

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💛 - Coveralls

[chenzhiguang]

There might be some other places using RegExp lookarounds, and I will fix them if there are any.

[kevmoo]

dart test -p safari now passes with these changes! thanks!

[chenzhiguang]

Nice, thank you too!