Allow injection grammars to specify embedded languages (#34327)

Fixes #34316

Enables contributed injection grammars to specify an embedded language for their injection targets:

```json
"grammars": [
  {
	"scopeName": "meta.embedded.css",
	"path": "./syntaxes/injection.json",
	"injectTo": [
	  "source.js",
	  "source.jsx",
	  "source.ts",
	  "source.tsx"
	],
	"embeddedLanguages": {
	  "meta.embedded.css": "css"
	}
  }
]
```