Add predicate for when running inside a VM

The is_vm() function is a predicate used to determine when we're running
inside a VM. It's currently extremely dumb: it checks for the presence
of a /vagrant directory.
Fix copy-n-paste comments in isnumeric()
Remove trailing whitespace
Remove unused function
Add weak_set() global function

Analogue to weak_export for setting shell variables
Make prompt birdtrack symbol a settable variable