Add completion for namespace access

[vinistock]

This PR implements completion candidates for namespace access (e.g.: Foo::). This scenario returns all constants and singleton methods accessible in the ancestor chain.

Note: there's an interesting behaviour in Ruby concerning inherited constants from Object and forward. They are not available when accessing namespaces even if present in the ancestor chain.

For example:

class Foo
end

Foo::String

You would expect Foo::String to be valid. Foo inherits from Object and String belongs to Object. However, Ruby actually cuts the ancestor chain short and does not include inherited constants for ancestor after Object. This is captured in the implementation and in tests.

[vinistock]

• Follow aliases for completion #600
• Provide completion for expressions inside call arguments #598
• Simplify completion tests with assertion macro #597
• Add method call with receiver completion #596
• Add completion for namespace access #595 👈 (View in Graphite)
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[Morriar]

I wonder if the singleton method ancestor walk should also stop at Object's singleton class?

The constant collection loop above breaks at Object to avoid surfacing inherited constants (e.g. Foo::String). Singleton methods inherited from Object's singleton (like new, allocate, superclass, etc.) would still be collected here since there's no equivalent cutoff. Is that intentional? If so, a short comment explaining the asymmetry would help.

[vinistock]

This is intentional because of Ruby semantics. Ruby prohibits you from referencing a constant inherited from Object (a.k.a. a top level level constant).

This crashes:

class Foo
end

# Foo inherits from Object and Object owns String
# but this is not allowed anyway
Foo::String

This is not the case for singleton methods. You can invoke singleton methods defined in Object or higher up the chain. I think the current behaviour does make sense.