os: add windows_volume function (#14721)

vlib/os/filepath_windows.v

@@ -0,0 +1,18 @@
+module os
+
+// windows_volume returns the volume name from the given `path` on a Windows system.
+// An empty string is returned if no Windows volume is present.
+// NOTE: An error is returned if the current operating system is not Windows.
+// Examples (on a Windows system):
+// ```v
+// assert os.windows_volume(r'C:\path\to\file.v') == 'C:'
+// assert os.windows_volume('D:') == 'D:'
+// assert os.windows_volume(r'\\Host\share\files\file.v') == r'\\Host\share'
+// ```
+pub fn windows_volume(path string) string {
+	volume_len := win_volume_len(path)
+	if volume_len == 0 {
+		return empty_str
+	}
+	return path[..volume_len]
+}