Add ./mach build --with-layout-2020

… with corresponding `layout` and `layout_thread` crates,
which for now do nothing.

components/layout_2020/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "layout_2020"
+version = "0.0.1"
+authors = ["The Servo Project Developers"]
+license = "MPL-2.0"
+edition = "2018"
+publish = false
+
+[lib]
+path = "lib.rs"
+test = false
+doctest = false
+
+[dependencies]
+style = {path = "../style", features = ["servo", "servo-layout-2020"]}

components/layout_2020/lib.rs

@@ -0,0 +1,3 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */