feat: voice channel groups

[amishshah]

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

Resolves #144.

VoiceConnections are now stored in a 2D map ({group_id -> {guild_id -> voice_connection}}).

The default global group ID is default - this is so the update is backwards compatible. Users may optionally specify their own group ID, e.g. the client ID, when creating their voice connections if they wish to run more than one bot in the process.

Also exposes two new methods:

• DataStore#getGroups() - get the full 2D group map
• DataStore#getVoiceConnections(group = 'default') - gets the map of guild IDs to voice connections for the given group

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• This PR changes the library's interface (methods or parameters added)

[codecov[bot]]

Codecov Report

Merging #147 (a122845) into main (36aad8d) will increase coverage by 0.90%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
+ Coverage   67.74%   68.65%   +0.90%     
==========================================
  Files          19       19              
  Lines         896      906      +10     
  Branches      214      221       +7     
==========================================
+ Hits          607      622      +15     
+ Misses        287      282       -5     
  Partials        2        2              

Impacted Files	Coverage Δ
src/joinVoiceChannel.ts	100.00% <ø> (+100.00%)	:arrow_up:
src/DataStore.ts	39.53% <100.00%> (+27.41%)	:arrow_up:
src/VoiceConnection.ts	85.82% <100.00%> (ø)

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