refactor: sub command => subcommand

[DaStormer]

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

I personally don't have a strong preference, but this is to remain consistent with the discord.js repository, which treats "subcommand" as one word.

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
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[vladfrangu]

Looks good

[DaStormer]

fb16424 should fix the failed checks; no idea why the file rename didn't commit before lol

[codecov-commenter]

Codecov Report

Merging #18 (fb16424) into main (34a1530) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #18   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          142       142           
  Branches        20        20           
=========================================
  Hits           142       142           

Impacted Files	Coverage Δ
src/interactions/slashCommands/Assertions.ts	100.00% <ø> (ø)
...nteractions/slashCommands/mixins/CommandOptions.ts	100.00% <ø> (ø)
.../interactions/slashCommands/SlashCommandBuilder.ts	100.00% <100.00%> (ø)
...eractions/slashCommands/SlashCommandSubcommands.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 34a1530...fb16424. Read the comment docs.