Implement resolveCompletionItem for hook descriptions

[johnbillion]

Implementing the resolveCompletionItem() method when registering a completion item provider means the construction of properties such as documentation get deferred until each individual completion item is selected.

This should improve performance as the extension won't need to construct the docs for every hook at the point where it provides the initial completion list.

Docs: https://code.visualstudio.com/api/references/vscode-api#CompletionItemProvider