Move atlas/Constants to browser

src/browser/renderer/atlas/Constants.ts

@@ -0,0 +1,9 @@
+/**
+ * Copyright (c) 2017 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+export const INVERTED_DEFAULT_COLOR = 257;
+export const DIM_OPACITY = 0.5;
+
+export const CHAR_ATLAS_CELL_SPACING = 1;