PathStrings: guard drive-letter normalization by platform

[hamdiakin]

Summary

• recognize drive-letter prefixes only when normalizing Windows paths
• keep the normalization core deterministic with an explicit drive-letter check
• cover Windows-style and Unix-style colon handling plus current-platform dispatch

Tests

• dotnet build --no-restore
• dotnet test --no-restore (262 passed)
• dotnet format --no-restore --verify-no-changes --include TruePath/PathStrings.cs TruePath.Tests/PathStringsTests.cs

Closes #219

[ForNeVeR]

Will you please take a look at the warnings in the code? See the nowarn-empty build job.

[ForNeVeR]

Thanks!