Interpret language client's support for deprecated fields in CompletionItem

[rcjsuen]

At the moment, CompletionItems are sent back to the language client with the deprecated field unfilled even though we claim we support it. Although dockerfile-language-service 0.0.5 was adopted when fixing #223, the service's corresponding capability must be set in order to get the field filled in.

We need to inspect the language client's support for deprecated fields in CompletionItem and then flag it as such to the service so that we get the right stuff returning back to the client.

This is technically a new feature but going to set this as a bug since we claimed we supported something when we actually didn't.