Language server notebook

[firelizzard18]

@h9jiang What do you think of an HTTP book style notebook for testing gopls commands? I'm imagining something where you put the command name and body into a notebook field and execute it, and the extension sends that command to gopls and presents the result and the execution time. It would make it easier to test features like the test explorer, but it would also provide a clean way for users to test things. For example with #3785 I could ask the user to execute gopls.packages via the notebook instead of via the CLI, which would eliminate the variable of whether something about the workspace or vscode was causing the issue (since it would run the command against the main instance of the language server). I'd be happy to contribute it myself (it wouldn't be my first notebook extension) but I want to check before submitting a CL. I thought about doing it as a separate extension, but the gopls instance is not exposed.