Clean up emoji compatibility code

[pento]

The code in wp-emoji-loader.js is fairly messy, and it's kind of annoying to have to remember to update several places when a new test is added (for example, we'll need a new test when Unicode 9 lands later in the year).

Let's make it easier to read, and fix how wp-emoji.js uses the settings.supports object, so that we don't have to keep updating it.

[pento]

35300.diff​ makes it a bit nicer to work with - we only need to add a new case in browserSupportsEmoji(), then add that corresponding test to the tests array.

[pento]

In 36816:

Emoji: Clean up the emoji browser support tests.

As new sets of emoji are added to Unicode, and browsers add support for them at random intervals, we'll inevitably need to add new tests to the emoji loader. This change makes it much easier to add new tests as they're needed.

Fixes #35300.

[pento]

In 36818:

Emoji: Fix some jshint warnings introduced in [36816].

See #35300.

[pento]

In 36927:

Emoji: Twemoji replacements weren't working in TinyMCE after [36816].

See #35300.

[pento]

In 37029:

Emoji: The everythingExceptFlag browser support flag, introduced in [36816], wasn't being initialised correctly.

See #35300.