Support for __proto__ as mapping key & anchor identifier

[eemeli]

JavaScript still supports the deprecated Object.prototype.__proto__ accessor for an object's prototype. This means that using __proto__ as a mapping key or an anchor name does unexpected things. For example:

const obj = YAML.parse('{ __proto__: [42] }') // Array {}
obj[0] // 42
obj.hasOwnProperty('__proto__') // false
JSON.stringify(obj) // '{}'

In other words, the value set for __proto__ would be assigned as the prototype of the generated object, unless you enabled the mapAsMap option. Using it as an anchor just wouldn't work.

Just to be clear, this isn't about prototype pollution: no existing prototypes get modified at any point, they just get created unexpectedly.

This PR allows for __proto__ as a mapping key or an anchor. The fixes should be robust enough that any other potential weirdnesses arising from inheriting from Object are averted as well.