Add new string assertions: BeParsableInto<T> and NotBeParsableInto<T>

[ITaluone]

Closes: #168

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
• The code complies with the Coding Guidelines for C#.
• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
• If the PR adds a feature or fixes a bug, please update the release notes with a functional description that explains what the change means to consumers of this library, which are published on the website.
• If the PR changes the public API the changes needs to be included by running AcceptApiChanges.ps1 or AcceptApiChanges.sh.
• If the PR affects the documentation, please include your changes in this pull request so the documentation will appear on the website.
  • Please also run ./build.sh --target spellcheck or .\build.ps1 --target spellcheck before pushing and check the good outcome

[coveralls]

Pull Request Test Coverage Report for Build 15660017692

Details

• 42 of 42 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 97.503%

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Totals
Change from base Build 15570311303:	0.01%
Covered Lines:	12514
Relevant Lines:	12694

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💛 - Coveralls

[cbersch]

That looks good! Thank you