Add parsed unknown flag to remaining arguments for a branch with a default command

[FrankRay78]

This PR is part of a series of CLI bug fixes/enhancements intended to be released prior to Spectre.Console V1.0

They should be individually reviewed and merged (and then the open ones rebased) in the following order, given they have been successively branched from each other (Rationale: earlier changes, particularly to CommandExecutor, are relied upon later). Separate PRs should aid the reviewer.

1. Add parsed unknown flag to remaining arguments for a branch with a default command #1660 (this one)
2. Correctly show application version; execution of command with version option #1663
3. Help output correctly decides when to show the version option #1664

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fixes #1607

• I have read the Contribution Guidelines
• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

• New unit tests to ensure unknown flags are added to remaining args
• Add strict parsing to almost all branches tests (an obvious omission from a prior PR)
• Fixed tests with incorrect validation that were in the main branch
• Significant improvement to the command line parsing to implement the above

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Please upvote 👍 this pull request if you are interested in it.

[FrankRay78]

Fully implemented and ready for review

Important: Please see the comment at the top of the issue description ahead of merging.