fix(GuildEmoji): check for changes to available in equals

[ckohen]

The available property was unchecked in GuildEmoji#equals causing updates to available to go uncached.
closes: #5200

I do not have an available guild where the state of available can change to test this on, please do test this if you can.

Status

• 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

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.