[add-axis] should include the `fallback_only` field

**Describe the issue**
The [`fallback_only`](https://googlefonts.github.io/gf-guide/axis-registry.html#the-fallback_only-field-affects-the-way-the-type-tester-surfaces-the-axis-control) is a metadata field required to indicate the UI how to present each axis in the UI.

The `add-axis` tool should include this field with a pre-defined `false` value, as it defines a slider for range axes, which are the most expected cases. The `true` option is used to represent binary and pseudo-boolean axes like [italic axis](https://github.com/googlefonts/axisregistry/blob/main/Lib/axisregistry/data/italic.textproto); however, the latter is expected to be avoided or rather exceptions.

**Screenshots**