[PR] Add option to load page builder functionality

By default, this is disabled.

* [x] Ignore builder specific files from upstream, these are copied over when required and not part of the spine parent theme.
* [x] Introduce a `spine_enable_builder_module` filter so that we can enable builder functionality with intent at the platform level.
* [x] Add the template used by the spine parent theme to display builder content.
* [ ] Exclude "Builder Template" from the list of available templates when it is not enabled.