Fix outer-dimensions schema

The current definition of the outer-dimension schema is invalid

"outer-dimensions": {
  "outer-dimensions-mm": {
    "$ref": "#/$defs/outer-dimensions"
  },   
  "outer-dimensions-cm": {
    "$ref": "#/$defs/outer-dimensions"
  },
  "outer-dimensions-m": {
    "$ref": "#/$defs/outer-dimensions"
  }
},

A better and functional one would be something like

"outer-dimensions": {
  "type": "object",
  "properties": {
    "openSCAD": { "type": "string", ...},
    "unit": { "enum" : ["mm", "m", "cm" ]}
},

This whole block could be a $ref.

@moedn What do you think? Is this kind of change possible?