Fire toolactivated after invoking execute #332

The imperative execute steps fire toolactivated before invoking the tool's execute callback.

Chromium and WPT both do it the other way around:

• Chromium dispatches the event after invoking the callback. That way listeners see the tool's effects, for example a filled-in declarative form. It dispatches nothing if the callback detached the document.
• executeTool-abort.https.html expects ['tool started', 'toolactivated', …].
• executeTool-detach-toolactivated.https.html expects no event after execute detaches the document.

Proposal: move the step after the call to execute, and add a guard:

 1. Let |toolPromise| be the result of [=invoke|invoking=] |tool|'s {{ModelContextTool/execute}} with
    |inputObject| and |options|.

+1. [=Fire an event=] named {{ModelContext/toolactivated}} at |targetDocument|'s [=Document/associated
+   ModelContext|associated <code>ModelContext</code>=], using {{ToolActivatedEvent}}, with its
+   {{ToolActivatedEvent/toolName}} attribute initialized to |tool|'s {{ModelContextTool/name}}.
+
 1. [=promise/React=] to |toolPromise|:

Related: #126, #245.