feat(editor): store real color values in edgeless (#9254)

### What's Changed!

* adds theme type: `ThemeSchema`
* adds default theme: `DefaultTheme`
* stores real color values

blocksuite/tests-legacy/snapshots/basic.spec.ts/basic-test-default.json

@@ -17,7 +17,10 @@
       "version": 1,
       "props": {
         "xywh": "[0,0,498,92]",
-        "background": "--affine-v2-edgeless-note-white",
+        "background": {
+          "dark": "#000000",
+          "light": "#ffffff"
+        },
         "index": "a0",
         "lockedBySelf": false,
         "hidden": false,