Import TextDecoder/TextEncoder Web Platform Tests

[TimothyGu]

See https://github.com/w3c/web-platform-tests/tree/master/encoding. Given that we already have a polyfill for testharness.js it should not be entirely difficult. However, care must be taken to comment out the portions of WPT that either only run in browsers, or are not currently supported by Node.js ('iso-8859-16' encoding), or are not supported by Node.js built with small-icu.

[jrasanen]

I'd be interested to help on this one

[kasimdoctor]

I'd be glad to help on this too! Is this already taken OR is there room for more than 1 person to work on this?

[TimothyGu]

@kasimdoctor This has already been worked on in #15743. Sorry for not updating the issue labels in time!

[gireeshpunathil]

associated PR is complete. Close?

[jasnell]

Closing :-)