fix(SlashCommandBuilder): allow subcommands and groups to coexist at the root level

[Khasms]

Please describe the changes this PR makes and why it should be merged:
As shown in #25 and as confirmed to me in DAPI, the root level of slash commands supports having both subcommands and subcommand groups, but builders does not. This PR removes the checks that prevented this and updates the relevant types. Also, the new return type name is quite lengthy so I'm open to suggestions on a more concise name.

fix #25

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 #26 (60e9fad) into main (2aecbe4) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #26   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          144       139    -5     
  Branches        21        19    -2     
=========================================
- Hits           144       139    -5     

Impacted Files	Coverage Δ
.../interactions/slashCommands/SlashCommandBuilder.ts	100.00% <ø> (ø)

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