Add top-level debug menu and keyboard shortcuts

A few notes -- 

- The keymappings are based on a combination of a survey of keymappings for other debuggers, with the goal of feeling at least reasonably familiar to folks who have used other debugging tools. I have tested them for conflicts on Chrome and Firefox on Windows, Mac, and Linux (and Safari on Apple). 

- A simple debugging guide and associated redirect link have been pushed to the repos for rstudio.com and rstudio.org.