feat: custom setting type

[did]

• declare custom setting type in Ruby
• refactor the app/frontend/editor/components/dynamic-form/dynamic-input.vue file so that we can support new dynamically new types
• merge potential custom JS components from the host app to the gem editor VueJS app. Use a Maglev config setting to know where to look at the custom JS code in the host app.
• generator to help to create a new custom type
• documentation

--- OLD ---

• declare custom setting type in Ruby
• allow to register an external VueJS component as a setting type
• improve the registration of the VueJS component
• expose some Editor functions: open/close modal
• generator if creating a new custom type
• documentation

[j-mcnally]

i want this, excited to see some work being done!