Enable the getter-return, no-dupe-else-if, and no-setter-return ESLint rules

All of these rules can help catch errors during development. Please note that only `getter-return` required a few changes, which was limited to disabling the rule in a couple of spots; please find additional details about these rules at:
 - https://eslint.org/docs/rules/getter-return
 - https://eslint.org/docs/rules/no-dupe-else-if
 - https://eslint.org/docs/rules/no-setter-return

src/core/cmap.js

@@ -394,7 +394,7 @@ class IdentityCMap extends CMap {
     return 0x10000;
   }
 
-  get isIdentityCMap() {
+  get isIdentityCMap() { // eslint-disable-line getter-return
     unreachable('should not access .isIdentityCMap');
   }
 }