Optimize json #include handling

[pvutov]

Good first issue because no Unity-specific programming is involved.

Our unit config files allow a c-style #include directive, which lets us break them up into small reusable pieces. Example:

{
    #include "DefaultMobility",
    #include "DefaultHeloArmor",
    "LabelIcon": "D",
    "CategoryKey": "HEL",
    "Name": "AH64D Apache",
    "Price": 20,
}

ConfigReader.cs has some custom code which replaces the #include lines with the content of the corresponding json files (e.g. DefaultMobility.json). This code does a lot of string operations of what will be hundreds of files, and is part of our startup loading work.

There are optimization opportunities:

• Smarter iteration over the files (currently if a file is included from two places, we will scan it twice instead of just once)
• Baking the results - for example, once the includes have been replaced everywhere, save the 'assembled' json files in a generated folder and use them for future launches. If the user changes the unit config, they will need to delete the generated folder.