Add Path functions (#505)

* Add Path functions

* Add unit tests

pkg/stdlib/path/join_test.go

@@ -0,0 +1,56 @@
+package path_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+	"github.com/MontFerret/ferret/pkg/stdlib/path"
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+func TestJoin(t *testing.T) {
+	Convey("When arg is not passed", t, func() {
+		Convey("It should return an empty string without error", func() {
+			out, err := path.Join(context.Background())
+
+			So(out, ShouldEqual, "")
+			So(err, ShouldBeNil)
+		})
+	})
+
+	Convey("Wrong argument", t, func() {
+		var err error
+		_, err = path.Join(context.Background(), values.NewString("/"), values.NewInt(0))
+
+		So(err, ShouldBeError)
+	})
+
+	Convey("Wrong argument within an array", t, func() {
+		var err error
+		_, err = path.Join(
+			context.Background(),
+			values.NewArrayWith(values.NewString("/"), values.NewInt(0)),
+		)
+
+		So(err, ShouldBeError)
+	})
+
+	Convey("Join(['pkg', 'path']) should return 'pkg/path'", t, func() {
+		out, _ := path.Join(
+			context.Background(),
+			values.NewArrayWith(values.NewString("pkg"), values.NewString("path")),
+		)
+
+		So(out, ShouldEqual, "pkg/path")
+	})
+
+	Convey("Join('pkg', 'path') should return 'pkg/path'", t, func() {
+		out, _ := path.Join(
+			context.Background(),
+			values.NewString("pkg"), values.NewString("path"),
+		)
+
+		So(out, ShouldEqual, "pkg/path")
+	})
+}