And do the same for Utf8View::code_point_offset_of(). Some of these
`VERIFY`s of the view's length were introduced recently, but they caused
the parsing of named capture groups in RegexParser to crash in some
situations.
Instead, allow indexing at the view's length: the byte offset of code
point `length()` is known, even though that code point does not exist in
the view. Similarly, we know the code point offset at byte offset
`byte_length()`. Beyond those offsets, we still crash.
Fixes 13 failures in test262's `language/literals/regexp/named-groups`.
By definition, the web allows lonely surrogates by default. Let's have
our string APIs reflect this, so we don't have to pass an allow option
all over the place.