feat: implement structures in mainlib

[Qjuh]

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

This is a very early proof-of-concept stage to discuss how @discordjs/structures might get implemented in next discord.js version.

Status and versioning classification:

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[codecov]

Codecov Report

Attention: Patch coverage is 1.13636% with 348 lines in your changes missing coverage. Please review.

Project coverage is 44.35%. Comparing base (3cff4d7) to head (e90deb7).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/next/src/managers/ChannelManager.ts	0.00%	53 Missing ⚠️
packages/next/src/Client.ts	0.00%	42 Missing ⚠️
packages/next/src/managers/CachedManager.ts	4.16%	23 Missing ⚠️
...s/next/src/structures/AnnouncementThreadChannel.ts	0.00%	19 Missing ⚠️
packages/next/src/structures/ForumChannel.ts	0.00%	19 Missing ⚠️
...ackages/next/src/structures/PublicThreadChannel.ts	0.00%	19 Missing ⚠️
...ackages/next/src/structures/AnnouncementChannel.ts	0.00%	18 Missing ⚠️
...ckages/next/src/structures/PrivateThreadChannel.ts	0.00%	18 Missing ⚠️
packages/next/src/structures/TextChannel.ts	0.00%	18 Missing ⚠️
packages/next/src/structures/StageChannel.ts	0.00%	17 Missing ⚠️
... and 10 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10983      +/-   ##
==========================================
- Coverage   45.23%   44.35%   -0.89%     
==========================================
  Files         307      327      +20     
  Lines       17189    17541     +352     
  Branches     1747     1763      +16     
==========================================
+ Hits         7776     7780       +4     
- Misses       9400     9748     +348     
  Partials       13       13              

Flag	Coverage Δ
next	1.13% <1.13%> (-18.89%)	⬇️

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