Handle the "Run" code lens

[rwols]

rust-analyzer shows a code lens with title "Run" at the main function of the program. This is supposed to build & run the executable.

One idea would be to check if Terminus is installed and spawn a shell with that.

[rwols]

Same for "Debug", although that's probably harder because we also need the presence of a debugger.

There's also a "Test" code lens. (finally, some interesting code lenses ;) )

[daggy1234]

10 attempts to solve this

[rchl]

Implemented through #10