Mod download improvements

[WiserTixx]

This PR adds multiple features relating to mod downloads.

1. With this PR the launcher will convert old style mods (without hash) to new style mods (with hash). This is so people don't have to re-download all of their mods after joining a server which was previously on a version below 3.6.0
2. This PR will save the last used date of mods in a JSON file in the caching directory. This will allow for smart deletion of cached mods

Left to-do:

• Implement some sort of way for lua to delete mods based on usage date

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