Expand Debug API for extensions

[weinand]

e.g. sessionStarted, sessionTerminated, sendCustomRequest, handleCustomEvent

[weinand]

new API:

/**
 * Start debugging by using either a named launch or named compound configuration,
 * or by directly passing a DebugConfiguration.
 * The named configurations are looked up in '.vscode/launch.json' found in the given folder.
 * Before debugging starts, all unsaved files are saved and the launch configurations are brought up-to-date.
 * Folder specific variables used in the configuration (e.g. 'workspaceRoot') are resolved against the given folder.
 * @param folder The workspace folder for looking up named configurations and resolving variables or undefined.
 * @param nameOrConfiguration Either the name of a debug or compound configuration or a DebugConfiguration object.
 * @return A thenable that resolves when debugging could be successfully started.
 */
export function startDebugging(folder: WorkspaceFolder | undefined, nameOrConfiguration: string | DebugConfiguration): Thenable<boolean>;

/**
 * The currently active debug session or `undefined`. The active debug session is the one
 * represented by the debug action floating window or the one currently shown in the drop down menu of the debug action floating window.
 * If no debug session is active, the value is `undefined`.
 */
export let activeDebugSession: DebugSession | undefined;

/**
 * An [event](#Event) which fires when the [active debug session](#debug.activeDebugSession)
 * has changed. *Note* that the event also fires when the active debug session changes
 * to `undefined`.
 */
export const onDidChangeActiveDebugSession: Event<DebugSession | undefined>;

/**
 * An [event](#Event) which fires when a custom DAP event is received from the debug session.
 */
export const onDidReceiveDebugSessionCustomEvent: Event<DebugSessionCustomEvent>;

/**
 * An [event](#Event) which fires when a debug session has terminated.
 */
export const onDidTerminateDebugSession: Event<DebugSession>;