Container type computation misses some types

[spoenemann]

Langium version: 2.0.0
Package name: langium

See the discussion here: #1055 (comment)

The current container type computation does not respect unassigned rule calls and actions when it determines a container relationship. In some cases (like the Keyword type of the grammar language) this leads to a container type that is too strict, i.e. it's missing some possible containers.

If this turns out to be a real problem in practice, we could improve the algorithm with something like this:

1. Traverse the parser rules looking for assigned rule calls. When we find one of the form A: ... b=B ...; we memorize the container A and go to rule B.
2. Add container A to the target type and all types mentioned by actions in the current rule.
3. If the current rule has unassigned rule calls, visit all of them by going to step 2. using the unassigned rule as current rule.