fix: tolerate explicit asset paths

[definitelyokay]

Description

This makes the tiled asset loader more forgiving if you explicitly specify the assets/images/ prefix, which is likely to happen if you're using something like flutter_gen to eliminate string typos.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• [x] I open this PR to the develop branch.
• [ ] I have added a description of the change under [next] in CHANGELOG.md.
• [x] I ran dart format --output=none --set-exit-if-changed . and has success.

Breaking Change

Does your PR require Flame users to manually update their apps to accommodate your change?

• [ ] Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md).
• [x] No, this is not a breaking change.

[RafaelBarbosatec]

Hi @definitelyokay ! Thanks for try contributing. Something wrong? I am available to help or answer any questions.

[definitelyokay]

@RafaelBarbosatec I closed because I wasn't sure how useful this is. This is a pretty easy thing to work-around. Feel free to merge if you think it's useful!

[RafaelBarbosatec]

Any contribution is welcome 🤗 Thank you so much!