util: implement WHATWG Encoding Standard API

Provide an (initially experimental) implementation of the WHATWG Encoding
Standard API (`TextDecoder` and `TextEncoder`). The is the same API
implemented on the browser side.

By default, with small-icu, only the UTF-8, UTF-16le and UTF-16be decoders
are supported. With full-icu enabled, every encoding other than iso-8859-16
is supported.

This provides a basic test, but does not include the full web platform
tests. Note: many of the web platform tests for this would fail by default
because we ship with small-icu by default.

A process warning will be emitted on first use to indicate that the
API is still experimental. No runtime flag is required to use the
feature.

Refs: https://encoding.spec.whatwg.org/
PR-URL: https://github.com/nodejs/node/pull/13644
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

tools/icu/icu-generic.gyp

@@ -30,15 +30,6 @@
       'type': 'none',
       'toolsets': [ 'host', 'target' ],
       'direct_dependent_settings': {
-        'conditions': [
-          [ 'icu_endianness == "l"', {
-             'defines': [
-                # ICU cannot swap the initial data without this.
-                # http://bugs.icu-project.org/trac/ticket/11046
-                'UCONFIG_NO_LEGACY_CONVERSION=1'
-             ],
-          }],
-        ],
         'defines': [
           'UCONFIG_NO_SERVICE=1',
           'UCONFIG_NO_REGULAR_EXPRESSIONS=1',