refactor: improve Shiki renderer initialization and theme handling

- Added disposal logic for the highlighter and renderer when handling Mermaid blocks.
- Enhanced the `ensureStreamMarkdownLoaded` function to register themes and update code more efficiently.
- Modified `initRenderer` to ensure proper initialization and disposal of resources.
- Updated theme registration to reactively watch for changes in props.themes.
- Cleaned up redundant checks and streamlined the code for better readability and performance.

Author: Simon-He95

package.json

@@ -76,7 +76,7 @@
     "katex": ">=0.16.22",
     "mermaid": ">=11",
     "shiki": "^3.13.0",
-    "stream-markdown": ">=0.0.6",
+    "stream-markdown": ">=0.0.8",
     "stream-monaco": ">=0.0.2",
     "vue": ">=3.0.0",
     "vue-i18n": ">=9"

playground/package.json

@@ -15,7 +15,7 @@
     "@tailwindcss/typography": "^0.5.19",
     "@vueuse/core": "^13.9.0",
     "katex": "^0.16.25",
-    "stream-markdown": "^0.0.6",
+    "stream-markdown": "^0.0.8",
     "stream-monaco": "^0.0.2",
     "vite": "^7.1.12",
     "vue": "^3.5.22",