refactor(formatters): Change :_: emoji name placeholder

[cobaltt7]

Please describe the changes this PR makes and why it should be merged:

Discord doesn't verify emoji names, meaning you only actually need to include a valid ID for an emoji to display properly. Discord.js uses _ as a fallback name when formatting emojis with just an ID. However, since _ is not a valid emoji name, if you were to parse that emoji, it would fail, which could be unexpected behavior.

Additionally, if you were to have multiple emojis with the _ name placeholder and invalid IDs (or if the bot doesn't have access to the emoji), they fail to render in Discord and the text between them becomes unintentionally italicized. For example, <:_:123678901234578> foobar <:_:123678901234578>

This is another unintentional side-effect of using :_: as a placeholder emoji name that end-users currently have to look out for.

To fix this, I changed the :_: placeholder to :emoji:. This way it is a valid emoji name that parses correctly and doesn't interfere with any other Markdown features. Originally, I was going to change it to :__:, which would parse correctly, but causes text between invalid emojis to be underlined.

In my opinion, parseEmoji should also be changed to properly support emojis that used :_: in the past, because even though it's not a valid emoji name, it does still render correctly, but people in the support server disagreed

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 38.02%. Comparing base (ed78e45) to head (4e90578).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10567      +/-   ##
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+ Coverage   38.01%   38.02%   +0.01%     
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  Files         239      239              
  Lines       15471    15466       -5     
  Branches     1353     1353              
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  Hits         5881     5881              
+ Misses       9575     9570       -5     
  Partials       15       15              

Flag	Coverage Δ
formatters	97.47% <100.00%> (ø)

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[Jiralite]

I personally would rather keep this as-is for the current minor version and make name required in the future. Thoughts? @vladfrangu @almeidx

[vladfrangu]

I personally would rather keep this as-is for the current minor version and make name required in the future. Thoughts? @vladfrangu @almeidx

🤷. We can also just have a fallback on unknown_emoji, and recommend users pass in the emoji name (but theres plenty times where you only have an ID but can still react). I personally don't think we need to make it required