types(ThreadChannel): fetchStarterMessage must return a Message<true>

[RedGuy12]

Please describe the changes this PR makes and why it should be merged:
Since threads are not supported in DMs, the Message returned by fetchStarterMessage must have inGuild = true.
Status and versioning classification:

• I know how to update typings and have done so, or typings don't need updating
• This PR only includes non-code changes, like changes to documentation, README, etc.

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Name	Status	Preview	Updated
discord-js	✅ Ready (Inspect)	Visit Preview	Sep 2, 2022 at 6:44PM (UTC)

[codecov]

Codecov Report

Merging #8560 (a00dac5) into main (3b7ba40) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head a00dac5 differs from pull request most recent head 4de1541. Consider uploading reports for the commit 4de1541 to get more accurate results

@@            Coverage Diff             @@
##             main    #8560      +/-   ##
==========================================
+ Coverage   86.35%   86.37%   +0.02%     
==========================================
  Files          86       87       +1     
  Lines        8598     8611      +13     
  Branches     1102     1107       +5     
==========================================
+ Hits         7425     7438      +13     
  Misses       1131     1131              
  Partials       42       42              

Flag	Coverage Δ
builders	100.00% <ø> (ø)
collection	100.00% <ø> (ø)
proxy	74.10% <ø> (ø)
rest	92.00% <ø> (ø)
utilities	100.00% <ø> (?)
voice	63.86% <ø> (ø)
ws	60.46% <ø> (ø)

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

Impacted Files	Coverage Δ
packages/actions/src/formatTag/formatTag.ts	100.00% <0.00%> (ø)

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[RedGuy12]

Tests are now passing.

[Jiralite]

Since threads are not supported in servers

You mean they are, right?

[RedGuy12]

Since threads are not supported in servers

You mean they are, right?

I meant in DMs