Enable grammar imports from node_modules

[msujew]

Right now, Langium only allows to import other grammars from a relative perspective. We might want to enable a behavior similar to JS/TS in which a non-relative import automatically leads to an import from node_modules. I.e.:

// current
import '../../node_modules/package/src/grammar';
// goal
import 'package/src/grammar';
// maybe even just (and add a way to discover grammars in the package.json)
import 'package';