fix(TypeScript): add CSSFunctions to CSSProperties typings (#392)

* Add CSSFunctions to CSSProperties typings

* TypeScript tests for nested css function prop

Author: quicksnap

test/glamorous.test.tsx

@@ -617,7 +617,7 @@ const BuiltinStyledWithFragment: JSX.Element = (
 const cssProps = {
   ":active": {
     color: "purple"
-  }
+  },
 };
 
 const BuiltinStyledWithCSSPseudoProps = (
@@ -626,3 +626,15 @@ const BuiltinStyledWithCSSPseudoProps = (
     <glamorous.A css={cssProps} />
   </div>
 );
+
+const nestedCssProps = [
+  { color: 'red', },
+  () => ({ fontSize: 123 }),
+  () => () => () => [{ border: '1px solid black'}],
+];
+
+const NestedStyleFunctions = (
+  <div>
+    <glamorous.Div css={nestedCssProps} />
+  </div>
+)

typings/css-properties.d.ts

@@ -1933,6 +1933,14 @@ export interface CSSProperties
     CSSPropertiesPseudo,
     CSSPropertiesLossy {}
 
-export type CSSPropertiesRecursive = CSSProperties | CSSPropertiesArray
+export type CSSPropertiesRecursive =
+  | CSSProperties
+  | CSSPropertiesArray
+  | CSSFunction
+
+// TODO: This could be made generic. Issue PR if you're so inclined!
+export interface CSSFunction {
+  (props: {}): CSSPropertiesRecursive
+}
 
 export interface CSSPropertiesArray extends Array<CSSPropertiesRecursive> {}