CLI arguments validation for single values that are overwritten

[BioPhoton]

If we use the same parameter multiple times yargs turns them into an array.

npx code-pushup --format=md --config=stdout turns the args into {format: ['md', 'stdout']}

This is wanted behaviour as forms is an array already.

In cases where we would need to overwrite CLI args that are no arrays this in introducing a wrong data structure.

npx code-pushup --config=./config.a.ts --config=./config.b.ts turns the args into {config: ['config.a.ts', 'config.b.ts']}

We should implement a check in the config middleware to take the last value only of the wrong array.