escapeWindowsPath() can produce ambiguous output and misses relative paths

In `src/main/java/org/apache/maven/shared/filtering/FilteringUtils.java`, lines 81–100:

The regex `^(.*)[a-zA-Z]:\\\\\\(.*)` only matches absolute Windows paths containing a drive letter. Relative paths with backslashes (e.g., `src\\main\\java`) are never escaped. The single/double backslash deduplication logic at line 90 (`if (val.indexOf('\\', end + 1) == end + 1)`) also means that `C:\\foo\\bar` and `C:\\foo\\\\bar` both produce the same output `C:\\\\\\\\foo\\\\\\\\bar` — information is lost about whether the original had single or double backslashes.

Start in src/main/java/org/apache/maven/shared/filtering/FilteringUtils.java, especially lines 81–100 and the condition at line 90. Trace the current handling of absolute and relative Windows paths using the examples in the issue, then verify that relative backslashes are escaped and single versus double backslashes remain distinguishable.

Written by the indexing model from the issue text.