Add themes support

Usage:

  const editor = new RibbitEditor({
    themes: [
        { name: 'dark', features: { sourceMode: false } },
        { name: 'minimal', tags: minimalTags },
    ],
    currentTheme: 'dark',
  });

The built-in theme is 'ribbit-default' and is always available.
Additional themes from the themes array are registered on top.

tsconfig.json

@@ -5,10 +5,10 @@
     "module": "ESNext",
     "moduleResolution": "bundler",
     "outDir": "dist",
-    "rootDir": "src",
+    "rootDir": "src/ts",
     "declaration": true,
     "sourceMap": true,
     "lib": ["ES2019", "DOM"]
   },
-  "include": ["src/**/*.ts"]
+  "include": ["src/ts/**/*.ts"]
 }