Add Workspace Notes plugin

[preetvadaliya]

Adds Workspace Notes — sticky notes for a Blockly workspace: draggable,
resizable, colour-coded paper with a title, an author and a stacking order.

Notes extend Blockly's own workspace comments, so dragging, resizing,
selection, keyboard navigation and undo all keep working unchanged. What is new
is the title, the colour, pinning, locking, stacking order, author and dates,
and a versioned save format.

Using it

import {WorkspaceNotes} from '@mit-app-inventor/blockly-workspace-notes';

const workspace = Blockly.inject('blocklyDiv', {toolbox});
new WorkspaceNotes(workspace).init();

Right-click the workspace to add a note, click a note's title to rename it, and
right-click a note to recolour, pin, lock, collapse or restack it.

Locking

A locked note is read-only. Its text, title and colour cannot be changed, and
it cannot be deleted, resized or copied. It shows a padlock in the title bar
and loses the bin beside it. It can still be moved, collapsed, restacked and
read.

Who may lock or unlock a note is the app's decision, through canToggleLock —
so a teacher can lock an instruction note that a student cannot unlock. That
gates the menu rather than the model, so it stops accidents and states intent;
anything that has to hold should be checked where the workspace is saved.

Pinning and locking stay independent: pinning decides where a note sits,
locking decides whether it can change.

Saving

Notes save under their own workspaceNotes key beside blocks, with a version
number so older files can be upgraded as they load. Only what differs from the
default is written, so files stay small and diff cleanly. Workspaces saved
before this plugin — with plain Blockly comments in them — still open, each
comment becoming a note. XML round-trips too.

What is in the PR

• The plugin under blockly-workspace-notes/, written in TypeScript, with 121
  tests
• Documentation in blockly-workspace-notes/docs/
• A CI workflow alongside the existing one

One thing worth flagging

This needs Blockly ^13.2.1, while main is on ^11.2.2. Nothing conflicts
inside the repo — it is a separate package with its own CI job — but the two
plugins' peer ranges cannot both be satisfied in a single app. Happy to
retarget this at update-to-blockly-13 if you would prefer.