feat(SlashCommandBuilder): add autocomplete

[Khasms]

Please describe the changes this PR makes and why it should be merged:
I figured out the types so I'm back. Adds support for autocompletion.

Status and versioning classification:

• Code changes have been tested, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)

[iCrawl]

Some tiny nits.

Maybe add test cases to the methods where you check if an error is thrown, should both choices and autocomplete be provided.

[codecov-commenter]

Codecov Report

Merging #53 (db1cf79) into main (0b6fb81) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #53   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines          264       276   +12     
  Branches        33        37    +4     
=========================================
+ Hits           264       276   +12     

Impacted Files	Coverage Δ
...nteractions/slashCommands/mixins/CommandOptions.ts	100.00% <ø> (ø)
...s/slashCommands/mixins/CommandOptionWithChoices.ts	100.00% <100.00%> (ø)

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[Khasms]

Some tiny nits.

Maybe add test cases to the methods where you check if an error is thrown, should both choices and autocomplete be provided.

My bad, I had already written the tests but forgot to commit them.

[iCrawl]

Can you also add some for the other 2 methods + toJSON?

[jqcktalks]

yes

[callumok2004]

Any ETA when this will be merged?