Allow to bring up a dialog with multiple inputs

## Feature request It would be nice if an extension could present a single dialog with multiple input fields. For example, the [New Project command in Ionide](https://github.com/ionide/ionide-vscode-fsharp/blob/a58b470dfd2a891eb46a545df0eb326fb0d93038/src/Components/Forge.fs#L142) asks for project directory and project name, and it would be nice to be able to input both at once in a single dialog. Another use case: I would like to implement the [Project Scaffold init script](https://github.com/fsprojects/ProjectScaffold/blob/master/init.fsx#L86) as another option in Ionide's New Project setup. It asks at least seven questions (more if you choose to let it run `git init` for you), and asking each question one at a time in separate dialogs makes for a less-than-ideal user experience. I'd like to be able to present all seven questions in a single dialog. ## Suggested API `showInputDialog(options: Map<string, InputBoxOptions|QuickPickOptions>, quickPickItems?: Map<string, string[] | Thenable<string[]>>): Thenable<Map<string,string>>` `options` is an ES6 Map whose keys are the names of the fields we're asking the user to fill in. The user won't see these names, but they will be used as the keys of the resulting Map object. Their ordering matters, since the order of the keys in the Map will be used as the order in which to present the fields in the interaction dialog. (If using an ES6 Map is not yet possible in VS Code, then `options` would be a plain old Javascript object. In that case, an additional `fieldOrder` parameter, an array of strings, would be required -- see below for suggested API if ES6 Map is not yet possible). The values of the `options` Map will be the InputBoxOptions or QuickPickOptions for that particular field in the interaction dialog. These will be treated exactly as they are in the `showInputBox` and `showQuickPick` functions. If any values of the `options` Map are `QuickPickOptions`, then the `quickPickItems` parameter is required. Its keys must ma…