PathTool.getRelativeFilePath: fix broken Windows drive-letter regex

[elharo]

The regex ^\\[a-zA-Z]: at lines 146 and 149 had two bugs:

1. The \\[ matched a literal [ character, not a backslash, because [ was escaped by \
2. String.matches() requires the entire string to match the pattern, but this regex was intended to match only the leading \C: prefix — so it never matched any real path

The Windows drive-letter normalization code was effectively dead. Paths like \C:\foo (produced by File.getPath() on Windows) were never getting their leading \ stripped, causing the drive-letter comparison logic at lines 154–175 to be skipped. This could produce incorrect relative paths or fail to detect mismatched drives (returning a path instead of null).

Fix: Replaced the broken regex with a simple character-level check:

if (toPath.length() > 2 && toPath.charAt(0) == '\\'
        && Character.isLetter(toPath.charAt(1)) && toPath.charAt(2) == ':')

Fixes #383