Provide an option to require explicit `var` declarations

[christoph_schlumpf]

I could open an issue (feature request) on github to add a complier flag like proposed by @samufi :

@samufi

Maybe a good middle ground for now would be to add a compiler argument --enforce-var (or --warn-on-missing-var) so that people building complex software can spot their potential bugs, and others can keep scripting.

Would the Modular team agree to open such an issue to discuss the details?