Add escapable prop to modals

[joshhanley]

The companion Flux Pro PR should be reviewed and merged before this PR: livewire/flux-pro#519

The Scenario

When rendering a modal that should require an explicit action before closing, there is currently no supported way to prevent Escape from closing it.

Setting :dismissible="false" prevents outside-click dismissal, and setting :closable="false" hides the close button, but pressing Escape still closes the modal.

<flux:modal
    name="mymodal"
    :dismissible="false"
    :closable="false"
>
    foo
</flux:modal>

The Problem

Flux Pro needs an internal disable-escape modal marker so the JavaScript layer can opt a modal out of browser Escape dismissal. That support is being added in livewire/flux-pro#519.

Once that internal marker exists, the public flux:modal component still needs an API for rendering it. Without a public prop, users cannot opt out of Escape dismissal through Flux Blade components.

The Solution

Add a separate escapable prop to flux:modal.

This keeps the modal closing controls aligned with the behaviours they already represent. dismissible is documented as controlling outside-click dismissal: “If false, prevents closing the modal by clicking outside.” closable is documented as controlling the close button: “If false, hides the close button.”

We considered expanding :dismissible="false" so it also prevented Escape dismissal. That would make the reported example work without a new prop, but it would also remove a useful distinction: users can currently prevent accidental click-away dismissal while still allowing Escape as a keyboard-driven way to leave the modal.

With :escapable="false", users can opt out of Escape dismissal explicitly. When this prop is false, flux:modal renders the internal disable-escape marker on ui-modal, which the companion Flux Pro PR handles at the JavaScript layer.

<flux:modal
    :dismissible="false"
    :escapable="false"
    :closable="false"
>
    ...
</flux:modal>

Fixes #2676