src: fix ParseEncoding

"utf-16LE" was parsed "UNKNOWN", this fixes to "UCS2"
"utf-buffer" was parsed "BUFFER", this fixes to "UNKNOWN"
"utf-16leNOT" was parsed "UCS2", this fixes to "UNKNOWN"

PR-URL: https://github.com/nodejs/node/pull/33957


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>