Check use-external-holder before union internal holders

[yejianfengblue]

Test params. Connector is external and trrs.

{
    "keys": {
        "columns": 7,
        "rows": 5,
        "thumb-count": "six",
        "last-row": "full",
        "switch-type": "mx-snap-in",
        "inner-column": "normie",
        "hide-last-pinky": false
    },
    "curve": {
        "alpha": 12,
        "pinky-alpha": 12,
        "beta": 36,
        "centercol": 4,
        "tenting": 15,
        "rotate-x": 180
    },
    "connector": {
        "external": true,
        "type": "trrs",
        "micro-usb": true
    },
    "form": {
        "stagger-pinky-y": -13.0,
        "thumb-cluster-offset-x": 6.0,
        "hotswap": false,
        "stagger-index-y": 0.0,
        "thumb-middle-left-tenting-y": 69.0,
        "thumb-top-left-offset-y": -16.0,
        "wire-post": false,
        "stagger": true,
        "stagger-ring-z": 0.0,
        "thumb-top-right-tenting-y": 69.0,
        "wall-thickness": 3.0,
        "stagger-middle-z": -6.5,
        "screw-inserts": true,
        "web-thickness": 7.0,
        "thumb-top-left-tenting-z": 69.0,
        "thumb-top-left-tenting-x": 69.0,
        "thumb-middle-left-offset-x": -53.0,
        "stagger-middle-y": 2.8,
        "thumb-middle-left-offset-z": -12.0,
        "thumb-middle-left-offset-y": -26.0,
        "height-offset": 10,
        "thumb-middle-left-tenting-x": 69.0,
        "wide-pinky": false,
        "thumb-top-right-offset-x": -15.0,
        "stagger-index-z": 0.0,
        "thumb-top-right-tenting-z": 69.0,
        "thumb-cluster-offset-z": 7.0,
        "custom-thumb-cluster": false,
        "thumb-top-right-tenting-x": 69.0,
        "stagger-ring-y": 0.0,
        "thumb-middle-left-tenting-z": 69.0,
        "thumb-top-left-tenting-y": 69.0,
        "thumb-top-right-offset-y": -10.0,
        "thumb-top-right-offset-z": -5.0,
        "thumb-top-left-offset-x": -35.0,
        "thumb-cluster-offset-y": -3.0,
        "thumb-top-left-offset-z": -2.0,
        "stagger-pinky-z": 6.0
    },
    "misc": {
        "keycaps": false,
        "right-side": true
    }
}

The left image is generated with current refaktor branch. The pro-micro-holder (I called L hook before) is useless. The additional cube is part of internal trrs holder, which should be cut before plug the external holder. The right image is generated with my code change. Check use-external-holder before union internal holder, just like check use-external-holder before difference internal holder hole and space.

Code snippet which checks use-external-holder before difference internal holder hole and space. https://github.com/ibnuda/dactyl-keyboard/blob/84e86a58582954f60f7844ef571f62e69b9fd02d/src/dactyl_keyboard/manuform.clj#L1598-L1605

[happysalada]

Change is looking good. I'll merge this for now and test soon. Thank you for your contribution!