Implement the `deno.cache` command and code action#5

The Deno language server expects the client (this extension) to implement a `deno.cache` command: > `deno.cache` - This is sent as a resolution code action when there is an un-cached module specifier that is being imported into a module. It will be sent with and argument that contains the resolved specifier as a string to be cached. > — https://github.com/denoland/deno/blob/main/cli/lsp/README.md#commands Basically, the Deno code action to cache an imported module uses that command and because it's not implemented yet it doesn't work when clicked: <img width="707" alt="Screen Shot 2021-08-19 at 11 22 00 am" src="https://user-images.githubusercontent.com/1754873/129992847-74643ac6-25af-4a3c-8447-e9f23e04b830.png"> Having a `Deno cache` command is nice to have anyway, so that a user can cache the imported modules in the focused editor via either a Nova menu bar option _**Editor → Deno → Deno cache**_ or the Nova command palette. I can get this working ok, but for some reason the `deno.cache` command despite being registered can't be triggered by the Deno cache code action, it results in a Nova extension console error: ``` Got a workspace/executeCommand request, but it is not implemented Method not found ``` Here is the LSP traffic when you click the code action that should trigger the `deno.cache` command… Request: ```json { "jsonrpc": "2.0", "id": 93, "method": "workspace\\/executeCommand", "params": { "command": "deno.cache", "arguments": [ [ "https:\\/\\/deno.land\\/x\\/oak\\/application.ts" ] ] } } ``` Response: ```json { "jsonrpc": "2.0", "id": 93, "error": { "code": -32601, "message": "Method not found" } } ``` It seems to be a Nova bug that I've [reported in the Nova developer forum](https://devforum.nova.app/t/lsp-method-workspace-executecommand-isnt-implemented/1125). I'll avoid adding the command until we can be sure of the right way to do so, so it can be triggered both by a user and via code actions.