Rename "CSS Scroll API" to "Scroll customization API"

As discussed at today's Houdini F2F, "CSS" is misleading because
scrolling isn't limited to CSS (eg. could apply to a page without
any CSS on it at all).  Also "customization" (or "extensibility")
is the key piece.  Will continue to bikeshed on the exact name.

Author: RByers

css-scroll-api/Makefile renamed to scroll-customization-api/Makefile

@@ -1,9 +1,9 @@
 <pre class='metadata'>
-Title:  CSS Scroll API Level 1
+Title:  Scroll Customization API Level 1
 Status: DREAM
 Group: houdini
-ED: https://drafts.css-houdini.org/css-scroll-api-1/
-Shortname: css-scroll-api
+ED: https://drafts.css-houdini.org/scroll-customization-api-1/
+Shortname: scroll-customization-api
 Level: 1
 Abstract:
 Editor: Rick Byers, rbyers@chromium.org