Fix config inconsistencies

[NicEastvillage]

• Renames cars[i].config to cars[i].config_file to be in line with other fields for files
• Renames mutator.boost to mutator.boost_amount

[VirxEC]

It was always mutator.boost_amount in the match config toml so that isn't a breaking change. But a lot of people currently have their tomls setup to use config instead of config_file and after this update they will all break - could we perhaps add config as a temporary alias to config_file in the toml parsing of core? Maybe log a deprecation warning too.

[VirxEC]

If that's done, the version could be v5.beta.4.3

[NicEastvillage]

Sure. That is a good idea.

[VirxEC]

Thanks, LGTM