feat(stubs): add stubs for RustClosure

[Xenira]

Fixes: #373

Description

Added basic stubs for rust closure. This will always be included if the closure feature is enabled.

No argument information, as that is not rly available, but better than having no stubs.

Checklist

Check the boxes that apply (put an x in the brackets, like [x]). You can also check boxes after the PR is created.

• I have read the contribution guidelines.
• I have added tests that prove my code works as expected.
• I have added documentation if applicable.
• I have added a migration guide if applicable.

❤️ Thank you for your contribution!

[coveralls]

Pull Request Test Coverage Report for Build 16273052792

Details

• 13 of 15 (86.67%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 22.552%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/describe/stub.rs	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/describe/stub.rs	1	7.69%

Totals
Change from base Build 16252634989:	0.2%
Covered Lines:	882
Relevant Lines:	3911

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💛 - Coveralls