Fix physics component

[arya3d]

I investigated the "physics" parameter issue in the .scn files
and I noticed that parameter is not being used anywhere
and the only reason we have had physics in objects is because we set it to true manually ( in app-manager.js )

so here's what I suggest we do :

1. put the "physics" parameter in the "components" array in the .scn file
2. make sure all of our scene files follow that rule
3. update our docs

and finally here's what the user experiences :

• If the physics component is not specified in the .scn file, we're going to enable physics for the object by default
• If the physics component is set to true we enable and if it's set to false we disable physics for that object

[avaer]

Didn't this cause breakage in physics on testing?

Additionally,

• needs to be ported to https://github.com/webaverse/compiler/
• scenes need to be updated? https://github.com/webaverse/scenes/