Implement forward compatible Clock-Interface

The stella-maris/clock package provides an interface based on the
currently proposed status of PSR20. Due to the inactivity of the PSR20 working
group this is a way to already provide interoperability while still
maintaining forward compatibility. When the current status of PSR20 will
be released at one point in time the stella-maris/clock package will
extend the PSR20 interface so that this package becomes immeadiately
PSR20 compatible without any further work necessary. In the long run the
stella-maris/clock package will then be marked deprecated so that people
can then use the PSR20 provided implementation.

Should the implementation of PSR20 change between now and a possible
release then this interface will still exist and a possible
implementation will need more code-changes anyhow so this interface will
still provide some way of interoperability.

The implementation of the PSR20 polyfill has been moved to the
stella-maris/clock interface which can be implemented independentyl from
PSR20 and might allow some interoperability before the working group of
the FIG manages to decide upon something.

composer.json

@@ -11,7 +11,8 @@
         }
     ],
     "require": {
-        "php": "~7.4.0 || ~8.0.0 || ~8.1.0"
+        "php": "~7.4.0 || ~8.0.0 || ~8.1.0",
+        "stella-maris/clock": "^0.1.4"
     },
     "require-dev": {
         "phpstan/extension-installer": "^1.1",
@@ -27,7 +28,6 @@
             "Beste\\Clock\\": "src/Clock"
         },
         "files": [
-            "src/psr-polyfill.php",
             "src/Clock.php"
         ]
     },
@@ -40,7 +40,10 @@
         "psr/clock-implementation": "1.0"
     },
     "config": {
-        "sort-packages": true
+        "sort-packages": true,
+        "allow-plugins": {
+            "phpstan/extension-installer": true
+        }
     },
     "scripts": {
         "test": [