Allow IEqualityComparer in string (Not)BeEquivalentTo assertion

[vbreuss]

Add an overload for StringAssertions.BeEquivalentTo and StringAssertions.NotBeEquivalentTo that accepts a IEqualityComparer<string>.

Details are discussed in #2364.
Fixes #2339.

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[coveralls]

Pull Request Test Coverage Report for Build 6586651679

• 34 of 34 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 97.399%

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Totals
Change from base Build 6586600692:	0.002%
Covered Lines:	11665
Relevant Lines:	11856

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💛 - Coveralls

[github-actions]

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It seems all right 👌

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[vbreuss]

The changes will be implemented with #2413