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

[vv-monsalve]

Describe the issue
The fallback_only 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; however, the latter is expected to be avoided or rather exceptions.

Screenshots

[vv-monsalve]

@m4rc1e we are about to add ~20 parametric axes in a batch. It would be idea if the above would be supported by the tool before of that.

[m4rc1e]

SGTM. Lemme do that now.

[vv-monsalve]

@m4rc1e friendly ping on this ;)