Support for executing debugger commands upon hitting a breakpoint

The DAP currently has support for three types of additions to breakpoints including conditions, hit count, and log messages.

interface SourceBreakpoint {
  line: number;
  column?: number;
  condition?: string;
  hitCondition?: string;
  logMessage?: string;
}

It would be nice if a user could attach commands to be automatically executed when a breakpoint is hit. This would be useful for some debuggers that have an extensive list of debugger commands that are more powerful than simply evaluating an expression such as GDB or LLDB.

The addition to SourceBreakpoint could be:

 /**
   * An optional list of commands that the backend will execute and log the output
   * (if any).
   */
  commands?: string[];

I know of at least 2 IDEs (IntelliJ and Xcode) that have support for executing any arbitrary debugger command(s) when a breakpoint is hit.

Start with the DAP SourceBreakpoint definition and its existing condition, hitCondition, and logMessage fields. Review the 13-comment discussion for decisions about the commands field and output behavior; done requires an agreed protocol design that maintainers accept.