Make API commands simpler to consume

Today, in `vsocde.d.ts` we define a generic command (which is actually a command and minimal UI) and a way to execute any command. Namely these two symbols * run any command: `export function executeCommand<T>(command: string, ...rest: any[]): Thenable<T | undefined>;` * define a command for later execution, e.g. in tree items, `export interface Command { title: string; command: string; tooltip?: string; arguments?: any[]; }` In the name of arbitrary commands this API is kept very generic. However, some commands are known, we call them API commands and they are currently only documented, see https://code.visualstudio.com/api/references/commands, offering no tooling. So, in addition the generic approach we could invest in some TypeScript gymnastics that bring some specific overloads, so `executeCommand` something like this would work: ```ts export interface ApiCommandMap { 'vscode.open': (resource: Uri, options?: OpenOptions, label?: string) => Promise<boolean>; 'vscode.diff': (left: Uri, right: Uri, options?: OpenOptions, label?: string) => Promise<boolean>; } export namespace commands { export function executeCommand<K extends keyof ApiCommandMap, T>(command: K, ...args: Parameters<ApiCommandMap[K]>): Promise<ReturnType<ApiCommandMap[K]>>; export function executeCommand<T>(command: string, ...rest: any[]): Promise<T | undefined>; } ``` However, I still struggle to find a nice way to reuse the `ApiCommandMap` to create specific overloads for the `Command` type. I have something like this in mind: ```ts export interface Command2<K extends keyof ApiCommandMap> { title: string; command: K; tooltip?: string; arguments: Parameters<ApiCommandMap[K]>; } ``` The bottom line is that I want to make sure that a `Command` uses the right arguments when using a certain command identifier.