Templates: allow replacing theme templates with custom templates

[liviopv]

What problem does this address?

The template replacing/swapping option only works with templates bundled in the theme. User might want to design their own templates and use them to replace theme defaults using the options shipped in recent GB versions and WordPress 6.5

What is your proposed solution?

The "Transform Into:" option available in the inspector sidebar for Templates should surface custom templates.

[annezazu]

Ah ha. Just ran into this with template parts and I assume it's the same problem. +1 to this cc @scruffian