fix: use strict mode in function which use this

Without "use strict", `this` will be set to `globalThis` which is
typically the window object. If certain properties are set on the
window object (e.g. "window.target"), this can cause problems for the
`css` and `styled` functions.

Instead, "use strict" ensures that `this` will only have a value if it
is explicitly bound, and otherwise will be `undefined`.

Closes cristianbote#545

src/css.js

@@ -7,6 +7,7 @@ import { getSheet } from './core/get-sheet';
  * @param {String|Object|Function} val
  */
 function css(val) {
+    'use strict'; // we want "this" to be undefined instead of being globalThis, which is typically the window object
     let ctx = this || {};
     let _val = val.call ? val(ctx.p) : val;
 

src/styled.js

@@ -18,6 +18,7 @@ function setup(pragma, prefix, theme, forwardProps) {
  * @param {function} forwardRef
  */
 function styled(tag, forwardRef) {
+    'use strict'; // we want "this" to be undefined instead of being globalThis, which is typically the window object
     let _ctx = this || {};
 
     return function wrapper() {