Fix DAP's completion request with new IRB's completor

[st0012]

In IRB 1.8.2, the completor's design and API has been changed and thus would break DAP server's completion request. This commit fixes the issue by introducing a Completor class that abstracts away the differences between the old and new completor.

cc @tompng

[st0012]

With ruby/irb#730, the request should not be broken anymore once it's released. And while it's still better not to use the deprecated InputCompletor, I think we can wait a while to see how the new API will evolve before updating it.

[ko1]

I'm not sure but also LSP may need completion feature so different library is another idea for completion?

[st0012]

You're right, LSP also needs completion feature and we're building one in ruby-lsp with a inhouse indexer, which we hope to extract into a gem when it's feature-completed.

But unlike a LSP server, IRB and debug both have access to binding objects and runtime information, which means it can get great completion result without going through the expensive indexing process. So I think it's still worth exploring a different completor for IRB and debug.