FEATURE: `Files::isAbsolutePath` and `Files::isRealPath`

[mhsdesign]

These new utility methods are needed by neos. And its best to place them into the utility with additional tests.

See

• https://github.com/neos/neos-development-collection/pull/4509#pullrequestreview-1703744742
• and https://github.com/neos/neos-development-collection/pull/4359#pullrequestreview-1506601606

Upgrade instructions

Review instructions

Checklist

• [ ] Code follows the PSR-2 coding style
• [ ] Tests have been created, run and adjusted as needed
• [ ] The PR is created against the lowest maintained branch
• [ ] Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• [ ] Reviewer - The first section explains the change briefly for change-logs
• [ ] Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions