Add option to ignore admin paths

Here's an updated patch. It addresses my review but also makes this patch D10 compatible.

Also this restores the test that was removed in #28

Probably still needs real tests though.

Hi @neclimdul ,Reviewed the patch on drupal version 9.5.3-dev. I think Sub-pathauto (Sub-path URL Aliases) Version: 8.x-1.x-dev Works with druapl versions 8 and 9. it is not compatible with D10. So i have tested the patch with drupal9. Patch works perfectly for me. Need RTBC+1

Needs re-roll

Reroll

Testing concern:
- The default value of `process_admin_routes` is TRUE. However, in the `/tests/src/Kernel/SubPathautoKernelTest.php b/tests/src/Kernel/SubPathautoKernelTest.php` we have `\Drupal::configFactory()->getEditable('subpathauto.settings')->set('process_admin_routes', TRUE)->save();` Instead of replicating the default setting, in the test it should start with default settings and then toggle the setting and confirm it is working as expected.

Improved testing. Does what Nick suggested plus a bit more because it doesn't look like there was any _actual_ testing of the new feature provided here.

woops... diff'd the wrong commit. This should be correct.

Also noticed there was a dead property on the path processor. @see interdiff.

Nice one @neclimdul. Will test this today.

Maybe we should use a generic solution ✨ Ignore specific paths RTBC

Like this we can ignore not only admin, but any path we want. It would be more flexible