Add a dummy NoneCharAtlas implementation

This adds the logic needed to switch between multiple char atlas
implementations, and an example second implementation to test it out
with.

This also offers an escape hatch if something goes wrong with the char
atlas implementation(s).

typings/xterm.d.ts

@@ -61,6 +61,20 @@ declare module 'xterm' {
      */
     enableBold?: boolean;
 
+    /**
+     * What character atlas implementation to use. The character atlas caches drawn characters,
+     * speeding up rendering significantly. However, it can introduce some minor rendering
+     * artifacts.
+     *
+     * - 'none': Don't use an atlas.
+     * - 'static': Generate an atlas when the terminal starts or is reconfigured. This atlas will
+     *   only contain ASCII characters in 16 colors.
+     *
+     * Currently defaults to 'static'. This option may be removed in the future. If it is, passed
+     * parameters will be ignored.
+     */
+    experimentalCharAtlas?: 'none' | 'static';
+
     /**
      * The font size used to render text.
      */