Inserter: Patterns tab state

[scruffian]

What problem does this address?

Since #61108, there are several conditions under which the patterns tab doesn't display patterns:

1. There is no block selected
2. The theme removes support for core block patterns: remove_theme_support( 'core-block-patterns' );
3. The patterns haven't loaded yet.

In each of these cases we show this message in the patterns tab:

What is your proposed solution?

We should treat each of these situations differently:

1. We should still show the patterns and just insert them at the end of the content
2. We show a message to explain why
3. We show a loading placeholder

It would be useful to have designs for these states. cc @jasmussen @richtabor

[jasmussen]

This issue feels very reminiscent of #60991, enough that if we can do this:

We should still show the patterns and just insert them at the end of the content

It would likely solve both. We might refine this to "nearest insertable location" rather than "at the end". But wouldn't that mainly be the case? And if we can solve this, do we need loading placeholders or messages?

[jasmussen]

CC: @jameskoster

[richtabor]

We could combine these into one no results found view:

There's already a spinner used when pulling results from the block directory.; we could reuse that UX pattern:

As aside, but the Inserter should probably not render a "no results found" and a spinner at the same time.