Create a JSON file that integrates the dictionaries from #433 and #434

[lars-reimann]

Is your feature request related to a problem? Please describe

So far #433 and #434 only created internal Python dictionaries. The user is not able to inspect these without a debugger.

Desired solution

Combine the dictionaries and create a JSON file. Desired output:

{
    "constants": {
        "sklearn/sklearn._config/config_context/assume_finite": {
            "target": "sklearn/sklearn._config/config_context/assume_finite",
            "defaultType": "boolean",
            "defaultValue": true
        },
        "sklearn/sklearn._config/config_context/working_memory": {
            "target": "sklearn/sklearn._config/config_context/working_memory",
            "defaultType": "string",
            "defaultValue": "bla"
        },
        "sklearn/sklearn._config/config_context/print_changed_only": {
            "target": "sklearn/sklearn._config/config_context/print_changed_only",
            "defaultType": "none",
            "defaultValue": null
        },
        "sklearn/sklearn._config/config_context/display": {
            "target": "sklearn/sklearn._config/config_context/display",
            "defaultType": "number",
            "defaultValue": "3"
        }
    },
    "unuseds": {
        "sklearn/sklearn.__check_build/raise_build_error": {
            "target": "sklearn/sklearn.__check_build/raise_build_error"
        }
    }
}

The user should be able to specify the path of the created file via a command line interface.