`consume_until(foo)` stops before foo, and so does
`ignore_until(Predicate)`, so let's make the other `ignore_until()`
overloads consistent with that so they're less confusing.
This parsing is already duplicated between LibJS and LibRegex, and will
shortly be needed in more places in those libraries. Move it to AK to
prevent further duplication.
This API will consume escaped Unicode code points of the form:
\\u{code point}
\\unnnn (where each n is a hexadecimal digit)
\\unnnn\\unnnn (where the two escaped values are a surrogate pair)
