[GeckoView] Reduce the size of the *built* viewer

Given that the GV-viewer isn't using most of the UI-related components of the default-viewer, we can avoid including them in the *built* viewer to save space.[1]
The least "invasive" way of implementing this, at least that I could come up with, is to leverage import maps with suitable stubs for the GV-viewer.

The one slightly annoying thing is that we now have larger import maps across multiple html-files, and you'll need to remember to update all of them when making future changes.

---
[1] With this patch, the built `viewer.js` size is 391 kB and `viewer-geckoview.js` is 285 kB.

web/ui_utils.js

@@ -74,6 +74,12 @@ const SpreadMode = {
   EVEN: 2,
 };
 
+const CursorTool = {
+  SELECT: 0, // The default value.
+  HAND: 1,
+  ZOOM: 2,
+};
+
 // Used by `PDFViewerApplication`, and by the API unit-tests.
 const AutoPrintRegExp = /\bprint\s*\(/;
 
@@ -852,6 +858,7 @@ export {
   AutoPrintRegExp,
   backtrackBeforeAllVisibleElements, // only exported for testing
   binarySearchFirstItem,
+  CursorTool,
   DEFAULT_SCALE,
   DEFAULT_SCALE_DELTA,
   DEFAULT_SCALE_VALUE,