feat(core): created linter for #426

[elijah-potter]

This PR introduces a rule that searches for incorrect usage of the contraction let's.

@ccoVeille or @hippietrail, before merging I'd like to add more test cases. If have some more, just shoot 'em below.

Closes #426.

[ccoVeille]

Let's me
Let's you
Let's them
Let's him
Let's it
Let's her
Let's us

Ugliest variants

Lets me
Lets you
...

[hippietrail]

Lets go and check if this lint let's us catch this class of errors.

Oh here's one in harper's source, so better than a contrived example:

    // Often the longest and the shortest words are the most helpful, so lets push
    // them first.

See #460

[ccoVeille]

Does Rust support the following notation as other languages do.

Suggested change

	CurrencyPlacement => true,
	LetUsRedundancy => true
	);
	CurrencyPlacement => true,
	LetUsRedundancy => true,
	);

It could avoid updating unrelated line on each rule addition

[elijah-potter]

Rust does, but this is actually one of our macros. I don't think it would be worth the effort to add that in.

[grantlemons]

turns out it's really easy

add $(,)? to the end of the macro signature

    ($($linter:ident => $default:expr),* $(,)?) => {

[elijah-potter]

I've revised it to cover more cases, like those discussed in #470. Let me know if there is anything else that should be added to this rule before I can merge it.