Add support for Template Activation

[mikachan]

We should add support for Template Activation, which is currently under an experiment in Gutenberg: WordPress/gutenberg#73252

When this is stabilised, it will likely land in WordPress 7.0, so we should make sure the plugin works with this new template functionality before then.

Tasks

• Update get_block_templates() calls to explicitly request inactive templates when needed (e.g., when clearing customisations or exporting all templates): Template Activation: Update get_theme_templates #798
• Modify clear_user_templates_customizations() and clear_user_template_parts_customizations() to include inactive templates: Template Activation: Update get_theme_templates #798
• Add activation state checking to template filtering logic by modifying should_include_template()
• Add options to control whether inactive templates are included in exports
• Consider how to handle duplicate templates (same slug, different IDs) when saving to theme files

[MaggieCabrera]

WordPress/gutenberg#66950 is the issue that explains the new functionality

[MaggieCabrera]

This is looking good to me. I am interested to know how inactive templates will be handled by WordPress if a theme decides to provide them, and how they are defined as active or not. I don't think this has been discussed, maybe this will not be supported, and a theme can only provide one template