feat(collect, expect): add .toBeAssignableTo() and .toBeAssignableWith() matchers

[mrazauskas]

This PR is adding .toBeAssignableTo() and .toBeAssignableWith() matchers.

Real world usage shows that it is useful to check assignability both ways (similarly to >= and =<).

@jdeniau Thanks for finding better name for .toBeAssignableWith().

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.03% (target: -1.00%)	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (e968247)	5274	5123	97.14%
Head commit (4de5877)	5335 (+61)	5184 (+61)	97.17% (+0.03%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#141)	61	61	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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