lib.d.ts: Add Encoding Standard

[saschanaz]

TypeScript Version: 2.5.x

Code

TextDecoder

Expected behavior: Type should exist

Actual behavior: It doesn't

Spec: https://encoding.spec.whatwg.org/

Matching PR: microsoft/TypeScript-DOM-lib-generator#300

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

[saschanaz]

Already here 😁 microsoft/TypeScript-DOM-lib-generator#300

[saschanaz]

This has been fixed.

[mhegazy]

still need to port the changes from Microsoft/TSJS-lib-generator