improve the static LumeElement.defineElement() method

[trusktr]

Improve the static LumeElement.defineElement() method so that it returns the same class if the class is not already used with an existing element name.

BREAKING: If you relied on the return value always being a new class, now the value may be the same class if the element is not yet defined. If you really need the previous behavior, you can change something like this,

class SomeEl extends Element {...}

const NewEl = SomeEl.defineElement('new-el')

to this,

class SomeEl extends Element {...}

class NewEl extends SomeEl {}
NewEl.defineElement('new-el')