docs: Improving Space Shooter tutorial by moving attributes to constructors

[erickzanardo]

Description

This PR improves the Space Shooter Tutorial as a whole by moving the setting of component's attributes from their onLoad method, to their constructors.

Once this is merged, I will do two additional follow ups

• Using SpawnComponent instead of TimerComponent to create bullets
• Adding passive hitboxes to bullets and explaining why.

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?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

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