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:

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
discord-js	Ready	Preview	Comment	Oct 24, 2025 3:56pm
discord-js-guide	Ready	Preview	Comment	Oct 24, 2025 3:56pm

[codecov]

Codecov Report

❌ Patch coverage is 1.13636% with 348 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.52%. Comparing base (0c1ff5e) to head (3bccab8).

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   44.34%   43.52%   -0.83%     
==========================================
  Files         315      335      +20     
  Lines       18155    18504     +349     
  Branches     1805     1821      +16     
==========================================
+ Hits         8051     8053       +2     
- Misses      10092    10439     +347     
  Partials       12       12              

Flag	Coverage Δ
next	1.13% <1.13%> (-18.94%)	⬇️

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