Fix double modal dialog when used in StreamField blocks

[fabinol]

@aweakley I've picked up a bug since the Wagtail Upgrade on IC Base. This is the fix suggested by Claude and it's description:

create_instance_selector_widget() was being invoked twice for widgets
rendered inside StreamField blocks: once directly by the telepath
widget definition (which needs the widget API back synchronously),
and again by the Stimulus instance-selector controller's connect(),
triggered because the widget root also carries
data-controller="instance-selector". Each call bound its own click
handler, so a single click on the chooser button opened two stacked
modal dialogs -- requiring two dismissals to close if nothing was
selected.

Cache the widget API on the widget root so a second initialisation
call for the same element is a no-op.

Could you review to make sure it isn't doing something insane.

I did a static override at static/instance_selector/instance_selector_widget.js in ic-base