Store stats as a system metadata file

[jqnatividad]

stats should produce a system metadata file, similar to how index creates an index file.

The stats metadata file will be stored in <FILENAME>.csv.stats similar to how index creates an index file at <FILENAME>.csv.idx.

When another command requires summary stats (schema, tojsonl, the WIP describegpt, etc.), it will check if the stats file exists and use that instead of compiling the stats anew.

[jqnatividad]

make it <FILENAME>.stats.csv so opening it with Excel and other tools will be easier as it will just be recognized as csv.

[jqnatividad]

Done with #902