feat: Allow for bodyDef and fixtureDefs to be prepared earlier

[l1553k]

Description

This change introduces a way to create BodyDef and FixtureDefs as early as in constructor - or even earlier. This allows to pass position and other body/fixture related parameters to constructor, and not have them stick around as this.initialThis or this.initialThat.

For example:

class OffroadWheel extends BodyComponent {
  OffroadWheel(
    Vector2 initialPosition,
    double radius,
  ) : super(
    bodyDef: BodyDef(position: initialPosition), type: BodyType.dynamic),
    fixtureDef: FixtureDef(CircleShape()..radius = radius),
  );
}

I added fixtureDef constructor argument for convenience.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• No, this PR is not a breaking change.

Related Issues

[l1553k]

@spydon we spoke about this on discord recently. Please provide some initial feedback; if you think this is going in the right direction then I will work on making it merge worthy.

[spydon]

Other than the analyze warnings and the comment I made I think it looks good!

[spydon]

I think you can skip this one and only have the list of fixture definitions

[spydon]

Just these small things left and I think we're ready to merge:

• Added docs in the flame_forge2d section
• A line about the new arguments in the BodyComponent dartdocs
• Some basic test

[l1553k]

• A line about the new arguments in the BodyComponent dartdocs

I'm sorry for my noob question, but what do you mean?

[spydon]

• A line about the new arguments in the BodyComponent dartdocs

I'm sorry for my noob question, but what do you mean?

The dartdocs are the ones that start with /// above the class, so if you could just add two lines saying something like:

/// You can use the optional [bodyDef] and [fixtureDefs] arguments to create the [BodyComponent]'s body without having to create the definitions within the component.

[spydon]

Fix line length
EDIT: Accidentally made a comment instead of a push through the web IDE.