Provide completion for expressions inside call arguments

[vinistock]

This PR adds completion for expressions inside of call arguments. This is a special case of expression completion. It includes everything that you would expect from expression completion + any keyword parameters that the user may refer to.

[vinistock]

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

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[Morriar]

Let's add a few tests showing we only consider the "first" definitions we indexed:

def foo(a, b, c); end # we show this one
def foo(x, y); end 

Also, are sure the indexing order for the same method in different document is stable?

# file1.rb
def foo(a, b, c); end # we show this one

# file2.rb
def foo(x, y); end 

We should always pick the same. Do we need to sort the definitions() by document URI?

[vinistock]

Added a test for this. We always return the same definition because the hashing of the ID is deterministic, which means that we find the same entry in the hashmap when iterating. I don't believe we need to sort.