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

.eslintrc

@@ -35,10 +35,12 @@
 
     // Possible errors
     "for-direction": "error",
+    "getter-return": "error",
     "no-async-promise-executor": "error",
     "no-cond-assign": ["error", "except-parens"],
     "no-constant-condition": ["error", { "checkLoops": false, }],
     "no-dupe-args": "error",
+    "no-dupe-else-if": "error",
     "no-dupe-keys": "error",
     "no-duplicate-case": "error",
     "no-empty": ["error", { "allowEmptyCatch": true, }],
@@ -51,6 +53,7 @@
     "no-irregular-whitespace": "error",
     "no-obj-calls": "error",
     "no-regex-spaces": "error",
+    "no-setter-return": "error",
     "no-sparse-arrays": "error",
     "no-unexpected-multiline": "error",
     "no-unreachable": "error",

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');
   }
 }

src/display/font_loader.js

@@ -106,15 +106,15 @@ class BaseFontLoader {
     unreachable('Abstract method `_queueLoadingCallback`.');
   }
 
-  get isFontLoadingAPISupported() {
+  get isFontLoadingAPISupported() { // eslint-disable-line getter-return
     unreachable('Abstract method `isFontLoadingAPISupported`.');
   }
 
-  get isSyncFontLoadingSupported() {
+  get isSyncFontLoadingSupported() { // eslint-disable-line getter-return
     unreachable('Abstract method `isSyncFontLoadingSupported`.');
   }
 
-  get _loadTestFont() {
+  get _loadTestFont() { // eslint-disable-line getter-return
     unreachable('Abstract method `_loadTestFont`.');
   }
 

web/interfaces.js

@@ -12,6 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint-disable getter-return */
 
 /**
  * @interface