Linter should warn if 'let'/'const' and 'var' are used in the same file.

[shicks]

The following should generate a linter warning:

let foo = 1;
var bar = 2;

[Dominator008]

We might as well go further and have a lint check that warns on any var usage.

[mashedcode]

@Dominator008 my guess would be that people would do s/var/let/ just to get rid of the errors and that would be just as terrible because there's a reason there's const. So please provide an option to turn that warning off if you do so.

[ChadKillingsworth]

There are definite cases for var. Specifically we only support typdefs as vars because we expect hoisting.