Add screenshot generation commands to justfile

[rfuest]

The simulator justfile contained some example related code, that should be moved here:

#----------
# Examples
#----------

# Generates a screenshot of an example
generate-example-screenshot example:
    @mkdir -p "{{screenshots_dir}}"
    # Generating {{example}} screenshot
    EG_SIMULATOR_DUMP="{{screenshots_dir}}/{{example}}.png" \
    cargo run --example {{example}}

# Generates screenshots of all examples
@generate-example-screenshots:
    #!/usr/bin/env bash
    set -e
    for example in examples/*.rs; do \
        just generate-example-screenshot $(basename "$example" .rs); \
    done