Fix: yoda treats negative numbers as literals (fixes #1571)

Negative numbers like `-1` are represented in the AST as a numeric `Literal` node with positive one as the value, wrapped in a `UnaryExpression` node with `-` as the prefix operator to negate the literal. This PR treats both positive and negative numbers as normal literals.

Because the range test exception validates ranges to ensure that the left end of the range <= the right end of the range, it was necessary to combine the negating wrapper and literal into a single negative literal node rather than simply checking for the presence of the negating wrapper so that the actual negative value is available.

This brings test coverage of `lib/rules/yoda.js` back up to 100%.

It also fixes a spelling error in the docs pointed out by @ljharb.
And as I look at it some more, I don't think the name of this function says what it does. In some cases is used like a check, in others it's used in an assignment. I think this approach needs some cleaning up to make what's happening more transparent.