Refactor: Implement REST namespace filter and use classes

[badasswp]

This PR resolves this issue.

Description / Background Context

At the moment, the REST namespace is pretty much hard-coded and leaves no room for users to use a customized namespace of their choice. This PR introduces a filter called cbtj_rest_namespace to allow for this customization. It also contains significant refactor of the plugin to use classes.

Testing Instructions

1. Pull PR to local.
2. Build correctly by running rm -rf node_modules && yarn start.
3. Observe that block import & export functionality work correctly without any regressions.
4. Proceed to add custom filter like so:

add_filter( 'cbtj_rest_namespace', function( $namespace ) {
    return 'my-custom-namespace/v1';
} );

5. Observe that endpoint now works with my-custom-namespace/v1 in the route path.