add emoji picker for reactions

[chrisirhc]

TODO:

• Tests
• Localization
• Add support for all emoji that aren't available by default in emoji_picker_flutter (e.g. symbols like *️⃣ , which are available in iamcal but not in emoji_picker_flutter) (Related: Use the official metadata as source  Fintasys/emoji_picker_flutter#153 )
• Filter away emoji that were already added by current user (similar to previous ThumbsUp behavior)
• Show 3-4 most recent emojis in the menu for reuse via final recentEmojis = await EmojiPickerUtils().getRecentEmojis();
• Clean up commit messages to fit style guide

Demo:
Screen_recording_20240315_110944 Zulip Emoji.webm

Partially addresses #388

[gnprice]

Thanks @chrisirhc for sending this! Sorry I didn't manage to look at it sooner.

There's a couple of key pieces of functionality this is missing that are part of Zulip's model of emoji:

• the list of known emoji from Unicode varies as the server gets upgraded;
• more critically, there are custom emoji that people have added in each Zulip realm, beyond those in Unicode.
• (Also there's one "Zulip extra emoji", which is a sort of built-in global custom emoji: :zulip:, whose glyph is the Zulip logo.)

Those are covered in #669, which I've recently largely completed.

The emoji_picker_flutter package which this pulls in is interesting, but AFAICT from the readme it looks like it wants to handle only Unicode emoji — not custom emoji.

So I'm closing this PR because I don't think the direction it's taking is one that will meet our needs.