build: migrate ng-dev config to .ng-dev directory (#37142)

Migrate to using .ng-dev directory for ng-dev configuration to allow
better management of the configuration using multiple files.  The
intention is to prevent the config file from becoming unruly.

PR Close #37142

.ng-dev/config.ts

@@ -1,4 +1,4 @@
-import {MergeConfig} from './dev-infra/pr/merge/config';
+import {MergeConfig} from '../dev-infra/pr/merge/config';
 
 // The configuration for `ng-dev commit-message` commands.
 const commitMessage = {

.pullapprove.yml

@@ -953,6 +953,7 @@ groups:
           '.circleci/**',
           '.devcontainer/**',
           '.github/**',
+          '.ng-dev/**',
           '.vscode/**',
           '.yarn/**',
           'dev-infra/**',