refactor(collection): fix/silence linter warnings

[almeidx]

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

collection.test.ts: line 22, col 3, Warning - Avoid callbacks. Prefer Async/Await. (promise/prefer-await-to-callbacks)
collection.ts: line 567, col 8, Warning - Use for…of instead of .forEach(…). (unicorn/no-array-for-each)

https://github.com/discordjs/discord.js/actions/runs/4461177910/jobs/7834819524#step:8:76

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes

[vercel]

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Name	Status	Preview	Comments	Updated
discord-js	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Mar 25, 2023 at 9:30AM (UTC)
discord-js-guide	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Mar 25, 2023 at 9:30AM (UTC)

[codecov]

Codecov Report

Merging #9266 (0a38475) into main (3bd7607) will decrease coverage by 0.01%.
The diff coverage is 97.05%.

❗ Current head 0a38475 differs from pull request most recent head d94cee3. Consider uploading reports for the commit d94cee3 to get more accurate results

@@            Coverage Diff             @@
##             main    #9266      +/-   ##
==========================================
- Coverage   83.57%   83.56%   -0.01%     
==========================================
  Files         100      100              
  Lines        9568     9570       +2     
  Branches     1101     1103       +2     
==========================================
+ Hits         7996     7997       +1     
  Misses       1533     1533              
- Partials       39       40       +1     

Flag	Coverage Δ
collection	99.88% <97.05%> (-0.12%)	⬇️
proxy	79.56% <ø> (ø)
rest	92.07% <ø> (ø)
ws	57.67% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/collection/src/collection.ts	99.88% <97.05%> (-0.12%)	⬇️

... and 1 file with indirect coverage changes

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	58
🟢 Accessibility	100
🟢 Best practices	100
🟢 SEO	100
🟠 PWA	70

Lighthouse ran on https://discord-js-git-fork-almeidx-collection-linter-cd5a86-discordjs.vercel.app/