fix: don't use globalThis when using 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, check that `this` has been set to something other than
`globalThis` before using it. This ensures that `this` will only have
a value if it is explicitly bound, and otherwise will be `undefined`.

Note: I tried to add "use strict"; directives where needed, but couldn't
get them to "survive" the build process -- they were removed. This fix
seems to work just fine.

Closes #545

src/css.js

@@ -7,7 +7,7 @@ import { getSheet } from './core/get-sheet';
  * @param {String|Object|Function} val
  */
 function css(val) {
-    let ctx = this || {};
+    let ctx = (this !== globalThis ? this : undefined) || {};
     let _val = val.call ? val(ctx.p) : val;
 
     return hash(

src/styled.js

@@ -18,7 +18,7 @@ function setup(pragma, prefix, theme, forwardProps) {
  * @param {function} forwardRef
  */
 function styled(tag, forwardRef) {
-    let _ctx = this || {};
+    let _ctx = (this !== globalThis ? this : undefined) || {};
 
     return function wrapper() {
         let _args = arguments;