LibJS: Implement UTF-16 surrogate pair concatenation without iteration

Performance of string concatenation regressed in a57e2f9. That commit
iterates over the LHS string to find the last code unit, to check if it
is a high surrogate. Instead, first look at the 3rd-to-last byte in the
UTF-8 encoded string to check if it is a 3-byte code point; then decode
just those bytes to check if we have a high surrogate. Similarly, check
the first 3 bytes of the RHS string to check if we have a low surrogate.

0 files changed, 0 insertions, 0 deletions