[Sub-Issue] Add fluent tween builders with typed .start() and optional class-based API

[stringdotjar]

Currently, the way to create a tween is to manually create a FlixelTweenSettings object. This can cause annoyance with how much is required just to build one tween. To simplify this, we need to create static builder classes that automatically constructs tweens for all types.

Requirements:

Fluent builder API

• Entry points return a builder; all configuration methods (e.g. addGoal, setEase, setType, setDuration) return the builder for chaining.
• The chain ends with .start(), which creates the tween, adds it to the global manager, starts it, and returns it.

Typed return

Support a class reference (e.g. FlixelTween.tween(FlixelPropertyTween.class)) so that .start() returns the concrete type (e.g. FlixelPropertyTween), enabling something like:

// Reflection tween example.
FlixelPropertyTween tween = FlixelTween.tween(FlixelVarTween.class)
  .setObject(...)
  .addGoal(...)
  .setDuration(1f)
  .setCallback(values -> {...})
  .start();

// Property tween example.
FlixelPropertyTween tween = FlixelTween.tween(FlixelPropertyTween.class)
  .addGoal(...)
  .setDuration(1f)
  .start();

// Numerical tween example.
FlixelPropertyTween tween = FlixelTween.tween(FlixelNumTween.class)
  .from(1f)
  .to(2f)
  .setCallback(num -> {...})
  .setDuration(1f)
  .start(); // Make sure that the start method throws an exception if from, to or the update callback isn't set!