[Fix] setLocale: do not allow prototype pollution

[ljharb]

📊 Metadata *

Bounty URL: https://www.huntr.dev/bounties/1-npm-yup

⚙️ Description *

This uses a null object as the base locale data, so it doesn't have a prototype to pollute.

💻 Technical Description *

Prototype Pollution refers to the ability to inject properties into existing JavaScript language construct prototypes, such as objects.
JavaScript allows all Object attributes to be altered, including their magical attributes such as proto, constructor and prototype.
An attacker manipulates these attributes to overwrite, or pollute, a JavaScript application object prototype of the base object by injecting other values.
Properties on the Object.prototype are then inherited by all the JavaScript objects through the prototype chain.

🐛 Proof of Concept (PoC) *

See tests.

🔥 Proof of Fix (PoF) *

See tests.

👍 User Acceptance Testing (UAT)

See tests. I also fixed the overarching test suite, which was only running a single test.

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