Add a "settings" or "..." button to TextFields and Items nodes

The settings button should open a Mantine Menu that:

- For TextFields, include a toggle that turns "Treat fields as templates" on or off. Turning it off makes sure that all fields escape braces {}.
- For TextFields, include a "Convert to Items node" and "Convert to Tabular Data" options that swap the current TF for an Items node/Tabular data node.
- Do the same for Items nodes, except with a "Convert to TextFields" button.