Implement rule "Prefer lighter formats for image files"

[utarwyn]

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Rule EC31 can be implemented in the plugin, for React/JSX.

Rule description in the ecoCode repository: https://github.com/green-code-initiative/ecoCode/pull/269

If you start this task during the challenge, I suggest you leave a comment with your team's name to let us know you've taken it. Then, you can link your PR so that we can easily keep track of the work you've done. Thank you and enjoy the challenge!