Align project structure with silverbullet-plug-template

- Rename deno.jsonc to deno.json and update build tasks
- Add PLUG.md for SB v2 installation
- Update .gitignore to include .plug.js files
- Update README.md with new installation instructions
- Simplify GitHub workflow

.vscode/settings.json

@@ -1,5 +1,5 @@
 {
   "deno.enable": true,
   "editor.formatOnSave": true,
-  "deno.config": "deno.jsonc"
+  "deno.config": "deno.json"
 }