feat: Add custom branding functionality with CSS and JSON configuration

- Introduced custom branding CSS styles in `custom-branding.css`.
- Created example JSON configuration for custom branding in `custom-branding.example.json`.
- Implemented branding configuration logic in `BrandingConfig.ts` to normalize and validate branding settings.
- Developed CSS loader to manage loading and unloading of custom stylesheets in `BrandingCssLoader.ts`.
- Added DOM rendering capabilities for branding elements in `BrandingDomRenderer.ts`.
- Defined types and interfaces for branding elements and configurations in `BrandingTypes.ts`.
- Included localization support for German in `de-de.js`.
- Added unit tests for branding configuration, CSS loader, and DOM renderer.
- Validated project structure and static assets with new validation scripts.

examples/custom-branding.example.json

@@ -0,0 +1,41 @@
+{
+  "schemaVersion": 2,
+  "enabled": true,
+  "debug": false,
+  "allowedCssHosts": [],
+  "cssfiles": [
+    {
+      "path": "~sitecollection/SiteAssets/branding/custom-branding.css",
+      "media": "all"
+    }
+  ],
+  "placeholdertop": {
+    "elements": [
+      {
+        "type": "section",
+        "attributes": {
+          "class": "custom-branding-banner",
+          "role": "region",
+          "aria-label": "Portalhinweis"
+        },
+        "children": [
+          {
+            "type": "strong",
+            "content": "Willkommen im Portal"
+          },
+          {
+            "type": "a",
+            "content": "Zur Startseite",
+            "attributes": {
+              "href": "~sitecollection/SitePages/Home.aspx",
+              "class": "custom-branding-link"
+            }
+          }
+        ]
+      }
+    ]
+  },
+  "placeholderbottom": {
+    "elements": []
+  }
+}