fix(types) adds types for mixin

[dimitropoulos]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

mixin is hardcoded to any for its argument and its return.

Issue Number: N/A

What is the new behavior?

mixin returns the proper type

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

I couldn't get the project started correctly locally (this is my first contribution to nestjs I think):

But I believe the second commit is correct in the changes it makes. I guess the CI will tell us.

[coveralls]

Pull Request Test Coverage Report for Build 7a100b4b-dc5f-446c-9d2f-5b3522b484e1

• 5 of 5 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.741%

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Totals
Change from base Build ae7ac136-ac70-432e-a182-46db11fc9598:	0.0%
Covered Lines:	5080
Relevant Lines:	5362

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💛 - Coveralls

[kamilmysliwiec]

LGTM

[brandon-leapyear]

✨ This is an old work account. Please reference @brandonchinn178 for all future communication ✨

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This does break the case of generating a mixin

// works in version 7, not version 8
const mkFilter = (...args) => {
  class MyFilter {
    // implement class using args
  }

  return mixin(MyFilter)
}

TS4025: Exported variable 'mkFilter' has or is using private name 'MyFilter'.

Is this no longer supported, then? Or should it be filed as a bug?