Use languageId as key in prism generator

[cdietrich]

changed to languageId to harmonize with textmate/vscode behaviour

```language-id
Hello World!
```

[cdietrich]

@msujew what is the current process of running this generator e.g. against the domain model example?
simply changing the language config and npm run watch does not seem to be sufficient

[msujew]

@cdietrich Running npm run langium:generate in the root folder will run the generator for the langium language and all example languages.

[cdietrich]

thx it works ;)