Adjust lastIndex to leading surrogate when inside a surrogate pair in unicode RegExp

[auvred]

22.2.7.2 RegExpBuiltinExec:

11. If fullUnicode is true, let input be StringToCodePoints(S). Otherwise, let input be a List whose elements are the code units that are the elements of S.

13.b. Let inputIndex be the index into input of the character that was obtained from element lastIndex of S.

See also:

• Unicode RegExp with index points trail surrogate in surrogate pair is not covered in the spec tc39/ecma262#128
• https://chromium-review.googlesource.com/c/v8/v8/+/6368822

[bellard]

applied