feat: move fieldType to widget when we use widgets as fields

[ecarreras]

This pull request involves refactoring the Field and Widget classes to improve code organization by moving the _fieldType property and its related logic from the Field class to the Widget class.

Refactoring changes:

• src/Field.ts: Removed the _fieldType property and its getter method from the Field class. Additionally, removed the logic that sets _fieldType based on props.type. [1] [2]
• src/Widget.ts: Added the _fieldType property and its getter method to the Widget class. Also, included the logic to set _fieldType based on props.type. [1] [2]