Condense path joins

[LPGhatguy]

Writing code like this:

        let plugin_dir = user_dir
            .join("AppData")
            .join("Local")
            .join("Roblox")
            .join("Plugins");

...results in creation of several intermediate PathBuf objects that are immediately thrown away. We should instead use join for the first time we do this, and then PathBuf::push for all other times. That will let us reuse the same PathBuf object instead of copying and recopying all of its data multiple times.

[Kampfkarren]

Oof, I thought Rust would optimize this.