Clean up char atlas constant imports/exports

- Ensures that everything is only defined in one place, and everything
  imports from that place.
- Moves CHAR_ATLAS_CELL_SPACING into a subdirectory of shared/ so that
  CharAtlasGenerator can pull from it.

This addresses the comments on
https://github.com/xtermjs/xterm.js/pull/1307/files/454fd4bfb1ab5c9ece

src/shared/CharAtlasGenerator.ts

@@ -4,6 +4,7 @@
  */
 
 import { FontWeight } from 'xterm';
+import { CHAR_ATLAS_CELL_SPACING } from './atlas/Types';
 import { isFirefox } from './utils/Browser';
 
 declare const Promise: any;
@@ -29,8 +30,6 @@ export interface ICharAtlasRequest {
   allowTransparency: boolean;
 }
 
-export const CHAR_ATLAS_CELL_SPACING = 1;
-
 /**
  * Generates a char atlas.
  * @param context The window or worker context.

src/shared/atlas/Types.ts

@@ -0,0 +1 @@
+export const CHAR_ATLAS_CELL_SPACING = 1;