Boot: Add conditional inspector rendering via route.inspector()

[lezama]

Related #70862

Summary

Adds support for a route.inspector() function in route_module that allows routes to conditionally show/hide the inspector panel based on route params or search parameters.

Usage example in route.tsx:

export const route = {
  inspector: ({ search }) => {
    // Only show inspector when items are selected
    return search.selectedIds?.length > 0;
  },
};

When the inspector function is not defined, it defaults to true (always show).
When it returns false, the inspector div is not rendered even if an inspector component is exported from the content module.

Also adds a RouteConfig interface to types.ts that documents all available route lifecycle functions (beforeLoad, loader, canvas, inspector).

Test plan

• Routes without an inspector function should continue to show the inspector (default true)
• Routes with inspector: () => false should hide the inspector panel
• Routes with inspector: ({ search }) => search.selectedIds?.length > 0 should conditionally show inspector based on search params

[github-actions]

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Co-authored-by: lezama <migueluy@git.wordpress.org>

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[youknowriad]

This is a great addition and something we're going to need for quick edit in the site editor. Thanks.