Add shell debugging and fix timeout issue

[samtstern]

Description

• Adds --inspect-functions capability to functions:shell (fixes Implement breakpoint debugging in the functions:shell command #1872)
• Changes timeout behavior when debugging is enabled (fixes Change timeout behavior when Functions emulator running in debug mode #1931)

Scenarios Tested

Was able to successfully:

• Set a breakpoint in a function with functions:shell
• Run an emulated function with debugging on for more than the default timeout, no error
• Run an emulated function with debugging off for more than the default timeout, got error

Used this test case for everything:

exports.helloWorld = functions
  .runWith({
    timeoutSeconds: 10
  })
  .https.onRequest((request, response) => {
    setTimeout(() => {
      response.send("Hello from Firebase!");
    }, 11 * 1000);
  });

Sample Commands

$ firebase functions:shell --help
Usage: firebase functions:shell [options]

launch full Node shell with emulated functions

Options:
  -p, --port <port>           the port on which to emulate functions (default: 5000)
  --inspect-functions [port]  emulate Cloud Functions in debug mode with the node 
                              inspector on the given port (9229 if not specified)
  -h, --help                  output usage information

[bkendall]

nit: let debugPort: number | undefined;

Setting something to undefined always feels dirty to me haha

[samtstern]

Agree but this is .js ... so I can't add types.

[abeisgoat]

lgtm, can't imagine anything going wrong

[samtstern]

I hope to use a screenshot of this comment in a P0 bug one day.