Code Completion: Respect completion context

[obiwan87]

In the current version, the code completion contributor suggests everything there is in scope. However, this might be too much or lead to nonsensical code in contexts where only types are expected.

TODO:
Filter the suggestion list such that it offers

• only types where such are expected
• the elements of an enum where that enum is expected
• the fields of a struct on the left side of struct initialization statements
• the variants of a union in a type assert
• the selectable identifiers in case a type constrained parameter is used with "."
• ... to be continued