feat: Add verbosity shortcuts (experimental) (#9481)

## The basics

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

## The details
### Resolves

Fixes part of https://github.com/RaspberryPiFoundation/blockly-keyboard-experimentation/issues/764
Fixes part of #9450 (infrastructure needs)

### Proposed Changes

Introduces support for two new "where am I?" shortcuts for helping to provide location context for users:
- `I`: re-reads the current selected block with full verbosity (i.e. also includes the block's field types with their values in the readout).
- `shift+I`: reads the current selected block's parent with full verbosity.

Note that this includes some functional changes to `Field` to allow for more powerful customization of a field's ARIA representation (by splitting up value and type), though a field's value defaults potentially to null which will be ignored in the final ARIA computed label. This seems necessary per the discussion here: https://github.com/RaspberryPiFoundation/blockly/pull/9470/files#r2541508565 but more consideration may be needed here as part of #9307.

Some limitations in the new shortcuts:
- They will not read out anything if a block is not selected (e.g. for fields and icons).
- They read out input blocks when the input block is selected.
- They cannot read out anything while in move mode (due to the behavior here in the plugin which automatically cancels moves if an unknown shortcut is pressed: a36f3662b0/src/actions/mover.ts (L166-L191)).
- The readout is limited by the problems of dynamic ARIA announcements (per #9460).

### Reason for Changes

https://github.com/RaspberryPiFoundation/blockly-keyboard-experimentation/issues/764 provides context on the specific needs addressed here.

### Test Coverage

Self tested. No new automated tests needed for experimental work.

### Documentation

No new documentation needed for experimental work.

### Additional Information

This was spun out of #9470 with the intent of getting shortcuts initially working checked in even if the entirety of the experience is incomplete.

core/field_checkbox.ts

@@ -116,10 +116,18 @@ export class FieldCheckbox extends Field<CheckboxBool> {
     this.recomputeAria();
   }
 
+  override getAriaValue(): string {
+    return this.value_ ? 'checked' : 'not checked';
+  }
+
   private recomputeAria() {
     const element = this.getFocusableElement();
     aria.setRole(element, aria.Role.CHECKBOX);
-    aria.setState(element, aria.State.LABEL, this.getAriaName() ?? 'Checkbox');
+    aria.setState(
+      element,
+      aria.State.LABEL,
+      this.getAriaTypeName() ?? 'Checkbox',
+    );
     aria.setState(element, aria.State.CHECKED, !!this.value_);
   }