fix(core): return type alias instead of string from css and cx

[Anber]

Motivation

Strictly typed css significantly simplified the creation of @linaria/eslint plugin which can check that there are no constant class names in user code.

const fooString = "foo";
const fooClass = css``;

<div className="foo" /> <!-- not ok -->
<div className={fooString} /> <!-- not ok -->
<div className={fooClass} /> <!-- ok -->
<div className={cx(condition && fooClass)} /> <!-- ok -->
<div className={cx(condition && "foo")} /> <!-- not ok -->
<div className={cx(condition && fooString)} /> <!-- not ok -->

Summary

css returns LinariaClassName instead of string.
cx returns LinariaClassName if every argument is LinariaClassName, otherwise it return string.