* fix utf8 -> utf16 decoding bug on surrogate pairs
This fixes https://github.com/protobufjs/protobuf.js/issues/1473
The custom utf8 -> utf16 decoder appears to be subtly flawed. From my reading it appears the chunking mechanism doesn't account for surrogate pairs at the end of a chunk causing variable size chunks. A larger chunk followed by a smaller chunk leaves behind garbage that'll be included in the latter chunk.
It looks like the chunking mechanism was added to prevent stack overflows when calling `formCharCode` with too many args. From some benchmarking it appears putting utf16 code units in an array and spreading that into `fromCharCode` wasn't helping performance much anyway. I simplified it significantly.
Here's a repro of the existing encoding bug in a fuzzing suite
https://repl.it/@turbio/oh-no-our-strings#decoder.js
* fix lint
* add test case for surrogate pair bug
Co-authored-by: Alexander Fenster <fenster@google.com>
