README: syntax highlighting (#839)

README.md

@@ -167,7 +167,7 @@ The filenames and paths for generated bundles in each format are defined by the
   "exports": {
     "require": "./dist/foo.js",        // CommonJS output bundle
     "default": "./dist/foo.modern.js", // Modern ES Modules output bundle
-  }
+  },
   "types": "dist/foo.d.ts"             // TypeScript typings directory
 }
 ```
@@ -184,7 +184,7 @@ Node.js 12.16+ adds a new "ES Module package", which can be enabled by adding `{
 This property [changes the default source type](https://nodejs.org/api/packages.html#packages_determining_module_system) of `.js` files to be ES Modules instead of CommonJS.
 When using `{"type":"module"}`, the file extension for CommonJS bundles generated by Microbundle must be changed to `.cjs`:
 
-```
+```jsonc
 {
   "type": "module",
   "module": "dist/foo.js",  // ES Module bundle