chore: Include discord.js in the user agent string

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
This includes discord.js in the user agent string, even if a custom appendix is supplied.

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

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
discord-js	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 1, 2023 10:20pm
discord-js-guide	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 1, 2023 10:20pm

[codecov]

Codecov Report

Merging #9267 (9dbef03) into main (e2f39cc) will increase coverage by 19.87%.
The diff coverage is 85.71%.

@@             Coverage Diff             @@
##             main    #9267       +/-   ##
===========================================
+ Coverage   59.92%   79.79%   +19.87%     
===========================================
  Files         214      123       -91     
  Lines       13721    10309     -3412     
  Branches     1245     1155       -90     
===========================================
+ Hits         8222     8226        +4     
+ Misses       5459     2043     -3416     
  Partials       40       40               

Flag	Coverage Δ
proxy	79.56% <ø> (ø)
rest	91.89% <85.71%> (-0.03%)	⬇️
website	?
ws	57.61% <ø> (ø)

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

Impacted Files	Coverage Δ
packages/rest/src/lib/utils/constants.ts	96.87% <83.33%> (-1.44%)	⬇️
packages/rest/src/lib/REST.ts	98.87% <100.00%> (ø)

... and 99 files with indirect coverage changes

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[almeidx]

As a side note, shouldn't /rest's default user agent URL point to the new docs (https://discordjs.dev/docs/packages/rest or something), or use @discordjs/rest@<version> in the version itself? Currently, it's set to DiscordBot (https://discord.js.org, 1.6.0)

[Jiralite]

From https://discord.com/developers/docs/reference#user-agent-user-agent-example, there doesn't seem to be a place to include @discordjs/rest—supposedly, the URL accounts for that. Changing the URL to the new documentation site would probably be best done in another pull request where we change everything over.

[vercel]

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