FIELD: Field dialog, FIELD command and the ATTDEF field buttons

A Field dialog with the fields the application evaluates and the
reference format choices, FIELD placing a field as multi-line text, the
attribute definition dialogs' field buttons, field attachment on commit,
system variable / file size / regional date providers for field
evaluation, and the codec patch with the field authoring API.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/command.rs

@@ -2628,6 +2628,12 @@ pub trait CadCommand: Send {
     /// Resume the command with collected rich text.
     fn on_editor_text(&mut self, _value: String) {}
 
+    /// A field (code and referenced objects) to attach to the entity this
+    /// command commits.
+    fn text_field(&self) -> Option<(String, Vec<Handle>)> {
+        None
+    }
+
     fn on_editor_display_height(&mut self, _height: f64) {}
 
     /// Called when the user clicks and `needs_entity_pick()` is true.