fix: Update datepicker css module to use :global() syntax

This replaces the CSS modules `composes` syntax that was previously in use.
This allows all of the CSS rules to make it to the build version of the component library.
Update contributing file to document preferred approach.

CONTRIBUTING.md

@@ -26,8 +26,14 @@ Components are organized in the [components](src/components/) directory into:
 - Follow React Aria styling conventions:
   - Default styles are in [src/components/styles/react-aria](src/components/styles/react-aria) (do not modify)
   - To customize React Aria styles:
-    1. Preferred: Use [`composes`](https://github.com/css-modules/css-modules/blob/master/docs/composition.md) to apply default styles within a CSS module. Relevant default CSS files must be imported into [theme.css](src/components/styles/theme.css)
-    2. Alternative: If composing isn't practical, copy and modify React Aria styles directly in the component's CSS Module
+    1. Preferred: Use `:global()` selector in your CSS module to combine React Aria styles with your local styles. For example:
+       ```css
+       .myComponent:global(.react-aria-Component) {
+         /* Your styles here will apply when both classes are present */
+       }
+       ```
+       Relevant default CSS files must be imported into [theme.css](src/components/styles/theme.css)
+    2. Alternative: If extending React Aria styles isn't practical, copy and modify React Aria styles directly in the component's CSS Module
     3. For global style overrides: Add to the bottom of [theme.css](src/components/styles/theme.css) (e.g., adding hover states or applying font properties using CSS variables)
 
 ### Documentation