easy screenshot plugin

[mockersf]

Objective

• Part of Add instructions for taking good screenshots for release notes bevy-website#2253
• Have an easy setup to take screenshots

Solution

• Make a new EasyScreenshotPlugin in the dev tools

Testing

• Add to any example

        .add_plugins(bevy::dev_tools::EasyScreenshotPlugin {
            trigger: KeyCode::KeyP,
            ..default()
        })

• Run the example with the feature bevy_dev_tools enabled
• press the P key
• screenshot! 🎉

[ChristopherBiscardi]

never thought I'd see an IIFE in Rust.

Copied the plugin into a random project and configured it. Works as expected.