AutolinkExtensionSyntax regex does not work on Safari

[jason-simmons]

dart-archive/markdown@49eefd21 added a regex in AutolinkExtensionSyntax that uses the (?<= and (?<! lookbehind operators.

These operators are not currently supported in the Safari browser.

See flutter/flutter#120632

[kevmoo]

FYI @chenzhiguang – if you have thoughts

[kevmoo]

This is unfortunate, but it's unclear the overall priority of parsing markdown in Safari. Obviously a Flutter web scenario we care about.

[chenzhiguang]

I will fix it later today