[Mainline Feature] Custom mod loader

[Goldenfield192]

This PR changes the way we load UMC mods.
Instead of relying on (Neo)Forge's @Mod snnotation and mods.toml, we can now use umc.json to scan mods in Fabric's way.
The definition:

{
    "modId": "immersiverailroading",  //Mandatory
    "displayName": "Immersive Railroading", //Mandatory
    "mainClass": "cam72cam.immersiverailroading.ImmersiveRailroading", //Mandatory
    "version": "1.10.0", //Mandatory
    "authors": [ //Optional
        "cam72cam"
    ],
    "description": [ //Optional
        "Use multiple lines to define your multi-line description!"
    ],
    "modURL": "", //Optional
    "license": "", //Optional
    "dependencies": { //Optional
        "1.17.1-forge": [ //Version-specific dependency, have a higher priority over defaults
            {
                "modId": "optifine", //Mandatory
                "versionRange": "(,)" //Mandatory
            }
        ],
        "default": [ //Fallbacks
            {
                "modId": "trackapi",
                "type": "required",
                "versionRange": "[1.1,)"
            }
        ]
    },
    "integration": { //Will be removed or refactored
        "repo": "git@github.com:TeamOpenIndustry/ImmersiveRailroadingIntegration.git",
        "branch": "%s",
        "path": "ImmersiveRailroadingIntegration"
    },
    "umc": { //Will be removed or updated
        "version": "latest"
    }
}