Add a lint to ban specific built-in functions

[nex3]

The random() function exists primarily for Sass demos to produce cool effects—it's pretty much always a bad idea to use in real stylesheets. We should add an (off-by-default) lint to forbid this function, whether from the global namespace or from sass:math.

cc @rambleraptor

[nex3]

Maybe a more general way to do this would be to have a lint that just takes a list of banned functions.

[kristerkari]

Would this rule from stylelint be enough? https://github.com/stylelint/stylelint/blob/master/lib/rules/function-blacklist/README.md

[nex3]

No, that rule isn't aware of Sass's module syntax.

[kristerkari]

No, that rule isn't aware of Sass's module syntax.

I see.

Maybe a more general way to do this would be to have a lint that just takes a list of banned functions.

This seems like a good option to add in addition to the function-blacklist rule.