[plugin] don't wrap promise in promise

[svenefftinge]

What it does

Fixes a bug where command execution result was wrapped in an extra promise.

How to test

Install a vscode extension that executes a command expecting a result.
E.g. built-in emmet runs unsupported vscode.executeDocumentSymbolProvider https://github.com/microsoft/vscode/blob/8cafaf5900ac9765a6946ce9c660da54846d9d07/extensions/emmet/src/defaultCompletionProvider.ts#L166

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

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