The `assertion-arguments` rule should validate message argument type #167

This currently passes t.deepEqual({}, {}, {});, which can make for some hard to debug issues. @gajus reported such issue on Gitter.

We should ensure that if message is specified, it's a string. We could implement inline literal validation first, as that's pretty easy.

We should also detect the type if it's a variable defined in the same scope:

const message = 'foo';

...

t.deepEqual({}, {}, message);

@jfmengels Many rules could use some kind of naive type inference. Maybe eslint-ast-utils could add something like that?

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