Core: Fixed greedy lookbehinds

[RunDevelopment]

This fixes a bug I found while working on #2029.

Any pattern like the following:

{
    pattern: /I don't need a lookbehind|(but) I do/,
    lookbehind: true,
    greedy: true
}

Will fail if the alternative without the lookbehind is matched.

This makes greedy lookbehinds consistent with normal lookbehinds, which already support this 'partial' lookbehind.