Add Path functions (#505)

* Add Path functions

* Add unit tests

pkg/stdlib/path/lib.go

@@ -0,0 +1,18 @@
+package path
+
+import "github.com/MontFerret/ferret/pkg/runtime/core"
+
+// RegisterLib register `PATH` namespace functions.
+func RegisterLib(ns core.Namespace) error {
+	return ns.RegisterFunctions(
+		core.NewFunctionsFromMap(map[string]core.Function{
+			"BASE":     Base,
+			"CLEAN":    Clean,
+			"DIR":      Dir,
+			"EXT":      Ext,
+			"IS_ABS":   IsAbs,
+			"JOIN":     Join,
+			"MATCH":    Match,
+			"SEPARATE": Separate,
+		}))
+}