Create checkProp.md

Adds a simple `checkProp` factory function for re-usable utility predicates.

The reason the predicate comes before the prop is to signify the predicate will be called with the prop, however I have no desire to make that a hard requirement. Order can easily be changed.

snippets/checkProp.md

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+### checkProp
+
+Given a `predicate` function and a `prop` string this curried function will then take an `object` to inspect by calling the property and passing it to the predicate.
+
+It summons `prop` on `obj` and passes it to a provided `predicate` function and returns a masked boolean
+
+```js
+const checkProp = (predicate, prop) => obj => !!predicate(obj[prop])
+```
+
+```js
+const lengthIs4 = checkProp(l => l === 4, 'length')
+lengthIs4([]) // false
+lengthIs4([1,2,3,4]) // true
+lengthIs4(new Set([1,2,3,4]) // false (Set uses Size, not length)
+
+const session = { user: {} }
+const validUserSession = checkProps(u => u.active && !u.disabled, 'user')
+
+validUserSession(session) // false
+
+session.user.active = true
+validUserSession(session) // true
+
+const noLength(l => l === undefined, 'length')
+noLength([]) // false
+noLength({}) // true
+noLength(new Set()) // true
+```