Improve `not` error messages about ow

[alexnoz]

@kainiedziela the project's structure has changed since the PR was opened, so it's slightly outdated, I'll revisit the PR and update it tomorrow.

[SamVerschueren]

From #71 (comment)

When I started implementing not, I was thinking about rephrasing messages automatically. I thought, If I always use to be, we could change that to not be. But then I had error messages where I couldn't use to be. So if we want clean messages for not, we might want to add an invertedMessage property to the validation object.

[sindresorhus]

I think we can automate most of it by creating helpers for the most common cases, like to be, and support an invertedMessage option for the few cases we can't automate.

---

Expected bar to be of type string but received type number

Could be automated to something like:

expectToBeOfType('bar', 'string', 'number');

[SamVerschueren]

Good idea! I'll take a look when I find some time.

[erdahuja]

@SamVerschueren @sindresorhus Can you help getting me started here. i want to start my first OS contribution.

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[alexnoz]

AFAIU in almost all cases it's enough to just put not after the first to, like Expected 'value' to not be/have/match ..., and for the cases where this approach won't work, e.g. min/max length, we can add invertedMessage, right? If so, I'm halfway there 🙂

[kainiedziela]

Any further work being done on this issue? The proposed PR doesn't pass tests. I'm inclined to pick this up.

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