Expose completion to Ruby API

[vinistock]

Easier reviewed by commit. This PR exposes completion to the Ruby API.

Relevant decisions

Method structure

I tried to have a unified API for completion, but it actually makes things quite confusing. Each completion scenario requires different inputs, so trying to merge everything makes for a very unintuitive API.

Also, C is not very elegant when handling optional data for an enum, so you end up with this weird implementation with a bunch of pointers that are only expected to not be null for certain scenarios. This is why I went with separate completion methods for each scenario.

Completion response

We could use the LSP structure as the response for completion. However, I think that ties us unnecessarily to the LSP protocol and there might be other useful cases for completion.

I went with trying to return as much data as possible for the entities:

• Declarations are returned as their own object. That allows us to show fully qualified name, signatures, file names, documentation and really any other information we would like
• Keyword parameters and keywords don't have much other than their name, but having distinct objects allows us to differentiate them for the LSP kind

[vinistock]

• Expose completion to Ruby API #683 👈 (View in Graphite)
• Add helper for freeing C string array #682
• Include keywords in completion candidates #681
• Add keywords to model #680
• main

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