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/renderer/atlas/Types.ts

@@ -6,7 +6,6 @@
 import { FontWeight } from 'xterm';
 import { IColorSet } from '../Types';
 
-export const CHAR_ATLAS_CELL_SPACING = 1;
 export const INVERTED_DEFAULT_COLOR = -1;
 export const DIM_OPACITY = 0.5;