Fix parsing default props

[normancroan]

This change addresses a problem I encountered where my component defines defaultProps containing values of type other than string.

ex:

	static defaultProps: TextAreaProps = {
		rows: 5,
		cols: 30,
		name: "Example",
		id: "Another Example",
	};

Storybook threw an error from parseProps.js because the value of defaultValueDesc.value wasn't a string but was passed to clearString which invokes .replace on the arg.

I’ve renamed this function to getClearStringForVal to better convey the expectation of receiving a “clear” string and to support the existing cases where this function receives values other than strings as arguments.