chore: add jshint eqeqeq for src/ files

Add jshint "eqeqeq" and "eqnull" rules (to allow == null comparisons).
Only adds it to files under src/, because we these files are unit-tested.
Files in src/ngLocale are excluded, because the code is imported.

Closes #14287

src/.jshintrc

@@ -1,6 +1,8 @@
 {
   "extends": "../.jshintrc-base",
   "browser": true,
+  "eqeqeq": true,
+  "eqnull": true,
   "globals": {
     /* auto/injector.js */
     "createInjector": false,

src/ngLocale/.jshintrc

@@ -2,6 +2,7 @@
   "extends": "../../.jshintrc-base",
   "bitwise": false, /* locale files use bitwise operators */
   "maxlen": false, /* locale files are generated from a 3rd party library that has long lines */
+  "eqeqeq": false,
   "browser": true,
   "globals": {
     "angular": false