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/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`.');
   }