fix: properly type `toJSON` methods

[suneettipirneni]

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

Adds return types for toJSON that correspond with discord-api-types

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

[codecov-commenter]

Codecov Report

Merging #34 (3695209) into main (2aecbe4) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #34   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        18    +3     
  Lines          144       233   +89     
  Branches        21        33   +12     
=========================================
+ Hits           144       233   +89     

Impacted Files	Coverage Δ
.../interactions/slashCommands/SlashCommandBuilder.ts	100.00% <ø> (ø)
...eractions/slashCommands/SlashCommandSubcommands.ts	100.00% <ø> (ø)
src/interactions/slashCommands/Assertions.ts	100.00% <0.00%> (ø)
...nteractions/slashCommands/mixins/CommandOptions.ts	100.00% <0.00%> (ø)
...s/slashCommands/mixins/CommandOptionWithChoices.ts	100.00% <0.00%> (ø)
src/messages/embed/Embed.ts	100.00% <0.00%> (ø)
src/messages/embed/Assertions.ts	100.00% <0.00%> (ø)
src/interactions/slashCommands/options/number.ts	100.00% <0.00%> (ø)

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