Add --launch-browser option to shiny run, and random port

[jcheng5]

With this PR, you can add --launch-browser to the shiny run command and it will pop open a web browser on successful launch.

It's also possible to ask Shiny to use a random port by passing --port 0. This will be helpful for RStudio Server and other multiuser scenarios.

Testing notes

• Tested that shiny run --reload --launch-browser ... only launches a browser once, not on each autoreload (i.e. saving app.py should cause existing browser window to autoreload but NOT for a new browser window to be launched)
• Tested that shiny run --launch-browser ... (i.e. without --reload) works; this uses a totally different codepath than --reload
• Tested that --port 0 assigns random port numbers every time
• Tested that --port 0 --reload keeps the same random port number for each autoreload
• Tested that --autoreload-port 0 also gives a random port number (do View Source on the app page, and look for data-ws-url

Plus: all of the above, on RStudio Server
Plus: all of the above, on VSCode on RSW (--launch-browser has no effect)
Plus: all of the above, on Windows in VSCode

Integration notes

This PR was designed to work well with RStudio Server and Workbench, but only on a branch that I'm still developing. Basically, the --launch-browser looks for the BROWSER environment variable; on my branch, RStudio Server sets this environment variable to a script that causes a new window to be opened by the IDE.

[cpsievert]

LGTM besides minor comments/nitpicks

[jcheng5]

@shalutiwari I did extensive testing of this, and additional testing will occur when we test this change in conjunction with the RStudio IDE integration.