Refactor: CSS handling (#2344)

* refactor(css): only load base styles, add css task, cleanup exports
* style(base): add edge label bg
* refactor(css): use css-utils
* feat(core): add Panel component
* refactor(background): cleanup
* refactor(css-handling): cleanup

examples/nextjs/postcss.config.json

@@ -0,0 +1,18 @@
+{
+  "plugins": [
+    "postcss-nested",
+    "postcss-flexbugs-fixes",
+    [
+      "postcss-preset-env",
+      {
+        "autoprefixer": {
+          "flexbox": "no-2009"
+        },
+        "stage": 3,
+        "features": {
+          "custom-properties": false
+        }
+      }
+    ]
+  ]
+}