chore: 🏷️ Use generic return type for all jest matchers

[pcbowers]

This better aligns with jest documentation: https://archive.jestjs.io/docs/en/23.x/expect#expectextendmatchers and ensures that types can work properly with other jest extension libraries like jest-chain.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/spectator/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

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
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Currently, when another library is used that modifies the return type of a matcher, all custom matchers added by spectator cannot be overridden in TypeScript because they do not use the R generic. This fixes that problem.

Issue Number: N/A

What is the new behavior?

All custom matchers now use the R generic instead of boolean.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[pcbowers]

@NetanelBasal What does the release schedule look like for this library? Is there an estimate of when this will be available in a given version?

[NetanelBasal]

Done