Tune LSP completions resolve capabilities to be more immediate

[SomeoneToIgnore]

Follow-up of #75
Closes #72
Part of rust-lang/rust-analyzer#18504

As requested in #75 (comment) , I've used the original PR's change, removed filterText, sortText and restored documentation field.

On top of that, I've added command to be resolved, as additionalTextEdits are resolved anyway and its a quite rare thing to use.
From https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#completionItem

/**
 * An optional command that is executed *after* inserting this completion.
 * *Note* that additional modifications to the current document should be
 * described with the additionalTextEdits-property.
 */
command?: Command;

[hrsh7th]

I appreciate your patience.
Thank you.