Feature: Generate MultiMC instance zip for Modpack

[Stealthii]

In a similar light to comp500/packwiz-installer#10, the boilerplate needed to create a functional MultiMC instance zip is relatively light, and something that could be handled within the packwiz toolset. Creating issue here as it's most relevant to the installer.

Steps required:

• In instance.cfg set PreLaunchCommand to "$INST_JAVA" -jar packwiz-installer-bootstrap.jar {{toml_url}}
• In mmc-pack.json set Game jars appropriately, based on what's defined in the TOML.
• Spit out a zip of these two files and the bootstrap.jar

This may be best implemented as a client-side web app that can be hosted statically like the documentation.

[comp500]

I'm going to move this to the packwiz CLI repo, since I do want to make this primarily a packwiz command (like the CurseForge/Modrinth export commands) and wouldn't be in scope for the installer itself, but something on the website would be useful too.

I'm planning to have something on the website for packwiz-vanilla-installer which would give you a JAR that autofills the pack URL (from a page like https://packwiz.infra.link/download/vanilla#https://raw.githubusercontent.com/Madis0/fabulously-optimized/main/Packwiz/1.17.1/pack.toml) - I could do a similar thing for MultiMC instances. So that the user doesn't have to download and locate the pack zip manually, it might be best to make this a server-side generated zip which can be directly pasted into the MultiMC import window.

Once that's done I'll recommend it over manually distributing a zip, since I think it's easier for users to trust packwiz than to trust each pack developer (and I plan to add a confirmation window listing the sources mods are retrieved from, so a user can determine whether to trust them).