[StimulusBundle] Handles Windows directory separator when normalizing controller names

[tamcy]

Q	A
Bug fix?	yes
New feature?	no
Issues	Fix #1422
License	MIT

This fixes the issue that controller name normalization doesn't work as intended in Windows which uses \ as the directory separator. In fact, without the fix, PHPUnit fails when running on Windows platform with the following error:

1) Symfony\UX\StimulusBundle\Tests\AssetMapper\ControllerMapGeneratorTest::testGetControllersMap
Failed asserting that two arrays are identical.
--- Expected
+++ Actual
@@ @@
     4 => 'hello-with-dashes'
     5 => 'hello-with-underscores'
     6 => 'other'
-    7 => 'subdir--deeper'
-    8 => 'subdir--deeper-with-dashes'
-    9 => 'subdir--deeper-with-underscores'
+    7 => 'subdir\deeper'
+    8 => 'subdir\deeper-with-dashes'
+    9 => 'subdir\deeper-with-underscores'
     10 => 'typescript'
 )

[weaverryan]

Thanks for this excellent PR @tamcy!