Add subcommand to run scripts, shiny apps, etc. in packages

From @hadley.

E.g. some logic to "run" a project:
https://github.com/rstudio/rsconnect/blob/26ec2c7ca8379cef9d139a85a2cdb62ef6db9ead/R/dryRun.R#L133-L151

Package scripts could be in the `exec` directory, and we could have
```
rig run pkg::foo
```
to run script `foo.R` and 
```
rig run <path>
```
to run a project.