Path normalization should not check drive letter on non-Windows

[ForNeVeR]

See this:

TruePath/TruePath/PathStrings.cs

Lines 176 to 191 in f410e9a

	/// <summary>
	/// Determines whether the specified source contains a drive letter.
	/// </summary>
	/// <param name="source">A read-only span of characters to be checked.</param>
	/// <returns>
	/// <c>true</c> if the source contains a drive letter (e.g., 'C:'); otherwise, <c>false</c>.
	/// </returns>
	private static bool SourceContainsDriveLetter(ReadOnlySpan<char> source)
	{
	if (source.Length < 2)
	{
	return false;
	}
	return source[1] == VolumeSeparatorChar && (uint)((source[0] | 0x20) - 'a') <= 'z' - 'a';
	}

It seems this will break for paths like C:/ or /: on Unix.

Guard this per OS. Best of all, make the OS or bool checkDriveLetter a parameter, to keep the function behavior independent of OS.

[hamdiakin]

I would like to work on this. I will make the drive-letter validation explicit and platform-independent, apply it only for Windows path normalization, and add focused tests for colon-containing paths on non-Windows platforms.