Accept Throwable for onRejected and expand return type for wait()

[Ndiritu]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	fixes #29
Documentation	if this is a new feature, link to pull request in https://github.com/php-http/documentation that adds relevant documentation
License	MIT

What's in this PR?

• Corrects the PHPDoc for wait() to handle the possible void and null return values based on the method's description.
• Uses the \Throwable interface instead of \Exception in onRejected callable parameter type
• Adds PHPStan configs and workflow checks to catch future issues early.

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix

To Do

• Should we consider using conditional return types for wait() e.g. @return ($unwrap is true ? T : null|void). This would eliminate further PHPStan failures for dependent libs, however this syntax requires PHPStan ^1.6.0. Would this be a better approach? At the moment, using T|null|void could cause further PHPStan failures for dependent libs. Would these further failures be considered a BC break?

[dbu]

1.6 is from april 2022, so i think that seems reasonable to expect.

lets add that and avoid further confusion for consumers phpstan checks.

[dbu]

thanks a lot!