Add --delay-startup argument to Ark

[lionel-]

Progress towards posit-dev/positron#740

Adds a new undocumented argument --startup-delay. It takes the path to a notification file to which the frontend writes when the debugger has been attached. Ark blocks until a change to the file is detected. After that, startup continues as normal.

[jmcphers]

This LGTM, but I wouldn't have expected an argument called --delay-startup to take a file argument. Maybe if the argument is numeric we could sleep that number of seconds instead, to support more traditional debugger scenarios?

[lionel-]

This LGTM, but I wouldn't have expected an argument called --delay-startup to take a file argument.

Good point. I've renamed the argument to --startup-notifier-file.

Maybe if the argument is numeric we could sleep that number of seconds instead, to support more traditional debugger scenarios?

Good idea. I think I'd implement this with a new --startup-delay argument to keep things simple. Otherwise a typo in the supplied argument that causes a parse failure will produce surprising behaviour instead of an error. Now done in last commit.