Update configuration file

The configuration file could be updated to

{
    // `column_span` only works for certain generators, so it's better to remove it
    // instead, replace it with a generic way to configure any generator
    "generator": "retain-lines", // or { name: "dense", "column_span": 70 }

    // a new option to apply rules only to a set of files
    "filter": "src/**/*.lua",

    // rename `process` to `rules`
    "rules": [ // rename 
        "compute_expression",
        { "rule": "convert_index_to_field" },
        { 
            "rule": "convert_index_to_field",
            // new option to apply this rule specifically for a set of files
            "filter": "src/library/**/*.lua" 
        },
    ]
}