feat: add Path.rel()

pathstr.v

@@ -55,6 +55,40 @@ pub fn (p Path) quoted() string {
 	return os.quoted_path(p)
 }
 
+// rel returns the relative path to the base path.
+pub fn (p Path) rel(base Path) Path {
+	sep := os.path_separator
+
+	target_parts := p.trim_right(sep).split(sep)
+	base_parts := base.trim_right(sep).split(sep)
+
+	mut common_idx := 0
+	for common_idx < target_parts.len && common_idx < base_parts.len {
+		if target_parts[common_idx] != base_parts[common_idx] {
+			break
+		}
+		common_idx++
+	}
+
+	mut result_parts := []string{}
+
+	up_count := base_parts.len - common_idx
+
+	for _ in 0 .. up_count {
+		result_parts << '..'
+	}
+
+	for i := common_idx; i < target_parts.len; i++ {
+		result_parts << target_parts[i]
+	}
+
+	if result_parts.len == 0 {
+		return '.'
+	}
+
+	return result_parts.join(sep)
+}
+
 // abs returns the absolute path.
 pub fn (p Path) abs() Path {
 	return Path(os.abs_path(p))