fix: Improve ARIA region handling. (#9485)

## The basics

- [x] I [validated my changes](https://developers.google.com/blockly/guides/contribute/core#making_and_verifying_a_change)

## The details
### Resolves

Fixes #9451

### Proposed Changes

Removes the ARIA `region` role for both flyouts and workspaces that are within mutators.

### Reason for Changes

The use of the `region` role only adds confusion and slightly messes up region announcements for screen readers. `generic` has been used instead since it's the default container role (e.g. for `div`) and seems sufficient for what needs to be described in this case.

Note that the delayed initialization for the flyout role is due to flyout initialization happening a bit later than its workspace DOM creation (so it doesn't seem possible to check for mutator status yet). There might be ways of doing this a bit more cleanly as part of #9307.

### Test Coverage

No automated tests are needed for this experimental change. Manual testing comprised of navigating between the main workspace, the main workspace's toolbox and flyout, and a mutator workspace and flyout to validate that no unusual region readouts were happening. The accessibility node tree was also analyzed to verify that `generic` is correctly being applied as the role for the mutator workspace and flyout.

### Documentation

No new documentation is needed for this experimental change.

### Additional Information

This doesn't fully resolve all region issues, but it resolves the main ones (especially when combined with #9483 for NVDA). The main remaining problem at this point is that the main workspace itself is usually not read out as a region and it's not clear why. I suspect it has something to do with focus manager and how it automatically moves focus, but I'm not entirely sure what specific mechanism is causing the problem since both toolbox and flyout do something similar and don't have the same issue (flyout is particularly noteworthy since it's a workspace in itself).

There may be some other focus oddities happening to cause the difference but, for now, this seems reasonable. If testing or user feedback find that the lack of consistent region readout is problematic for the main workspace then a new issue can be opened and investigated separately.

Author: BenHenning

core/flyout_base.ts

@@ -338,6 +338,9 @@ export abstract class Flyout
   init(targetWorkspace: WorkspaceSvg) {
     this.targetWorkspace = targetWorkspace;
     this.workspace_.targetWorkspace = targetWorkspace;
+    if (this.targetWorkspace.isMutator) {
+      aria.setRole(this.workspace_.getFocusableElement(), aria.Role.GENERIC);
+    }
 
     this.workspace_.scrollbar = new ScrollbarPair(
       this.workspace_,

core/utils/aria.ts

@@ -54,6 +54,7 @@ export enum Role {
   COMBOBOX = 'combobox',
   SPINBUTTON = 'spinbutton',
   REGION = 'region',
+  GENERIC = 'generic',
 }
 
 /**

core/workspace_svg.ts

@@ -762,14 +762,19 @@ export class WorkspaceSvg
     });
 
     let ariaLabel = null;
+    let role: aria.Role | null = null;
     if (this.isFlyout) {
       ariaLabel = 'Flyout';
+      // Default to region, but this may change during flyout initialization.
+      role = aria.Role.REGION;
     } else if (this.isMutator) {
       ariaLabel = 'Mutator Workspace';
+      role = aria.Role.GENERIC;
     } else {
       ariaLabel = Msg['WORKSPACE_ARIA_LABEL'];
+      role = aria.Role.REGION;
     }
-    aria.setRole(this.svgGroup_, aria.Role.REGION);
+    aria.setRole(this.svgGroup_, role);
     aria.setState(this.svgGroup_, aria.State.LABEL, ariaLabel);
 
     // Note that a <g> alone does not receive mouse events--it must have a