feat(16.2-01): compile, decode and load plugin field types in cabana

- compileFieldTypes builds a per-boot field type registry (alias, core and
  cross-plugin collisions, element prefix, assets, option keys) passed down
  the compile functions; settings and relation forms refuse plugin aliases
- fields.yaml accepts a plugin alias with the general keys plus the type's
  options (typeOptions, element, translatable in the schema); CheckOptions
  runs at boot; CompileFormWithTypes for plugin tests
- save decodes plugin values inside the transaction (FormFieldError is a
  422, other errors opaque 500) and answers meta.notices (FieldNotice) only
  when a type reported something; show and save serve values through Load
- GET /field-types lists the types with versioned asset URLs; OpenAPI,
  schema.d.ts, README and docs/backend/field-types.md updated

docs/backend/forms.md

@@ -102,7 +102,7 @@ for _, f := range form.Fields {
 | `mltextarea` | A multi-line text field with one editor per locale; it takes `size` as `textarea` does. See [Markdown and multilingual fields](#markdown-and-multilingual-fields). |
 | `mlmarkdown` | A markdown field with one editor per locale; see [Markdown and multilingual fields](#markdown-and-multilingual-fields). |
 
-The WinterCMS widgets that are not in this list (the rich editor, the code editor, the color picker, the media finder, the repeater, the tag list and the others) are not provided. A field with one of those types stops the start-up.
+The WinterCMS widgets that are not in this list (the rich editor, the code editor, the color picker, the media finder, the repeater, the tag list and the others) are not provided. A field with one of those types stops the start-up, unless a plugin registers that alias as its own field type: plugins add field types with a custom element and Go code that decides what is stored; see [Plugin field types](field-types.md).
 
 ### Field options