Support CURRENT_VERSION file - could become the new norm?

Right now it seems to be an edge case from one modpack maker (World of Dragons/Winter is Here) but could become a common thing if it catches on. Instead of bundling the Forge Installer JAR they provide a CURRENT_INSTALLER file which has the URL to the relevant JAR.

I got it working by manually extracting the server files (rather than using `TYPE=CURSEFORGE`) and using `TYPE=FORGE` while providing the `FORGE_INSTALLER_URL` as the contents of the CURRENT_INSTALLER file.

Looking for one of an array of files (let's assume the Modpack community doesn't want to follow a single standard) and using that as the installer URL to download would be the most apt approach

Start by tracing the existing TYPE=CURSEFORGE and TYPE=FORGE handling, including how FORGE_INSTALLER_URL is obtained. Compare that flow with the CURRENT_INSTALLER file described in the issue and determine how candidate files should be selected. Done means the supported file-based installer URL flow works for the stated modpack case without requiring manual extraction.

Written by the indexing model from the issue text.