Help with reducing package size

[Vrixyz]

• Following feedbacks from Large filesize (1MB of raw WASM) #269 and Compress size experiments Vrixyz/rapier.js#3

Checklist:

• feature gate serialization
• feature gate debug render
• make it easier to tweak those options
  • code cleanup to only support file config
  • sibling documentation at Chapter on javascript size guidelines rapier.rs#138
• test it more

[LeXXik]

One other possibility for reduction could be reviewing the modules of Rust std being used. In some cases only a fraction of a module functionality is used and it is then beneficial to create a custom one just for the specific use of the library. This will prevent a complete module being pulled into the build, saving some space. In some cases it can help shaving off another 15-20%.