Use @pragma('vm:awaiter-link') for better stacktraces

[codedbycurtis]

Added the @pragma('vm:awaiter-link') annotation to each instance of Completer.
Closes #2291.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

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[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
Overall Coverage	🟢 84.46%	🟢 84.46%	⚪ 0%

Minimum allowed coverage is 0%, this run produced 84.46%

[AlexV525]

Could you also add tests to make sure stack traces have been improved?

[ueman]

Tests should be fairly straightforward to add. Just make the various cases throw an exception and call toString() on the stacktrace. Also make sure the stacktraces are actually better than before.

The ones in the test suite probably don't need tests, though

[AlexV525]

@codedbycurtis Still going to work on this?

[codedbycurtis]

@AlexV525 Apologies for the lack of updates on this. I tried to implement these tests a few weeks ago but was having trouble getting the tests to run with melos (something regarding invalid filters so probably just some obscure configuration issue on my end), and have not had the time recently to look into the issue. I am happy enough if someone else wants to take over and look into getting these tests added.