Report overall completeness in pyrefly report #2239
Description
Describe the Bug
We should output an overall completeness score and also a per-file completness score. For now, this can just be the percentage of parameters/returns that are annotated, rounded to 2 or 3 decimal places.
If anyone picks up the issue, please comment below w/ the proposed output format before starting implementation work
Sandbox Link
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(Only applicable for extension issues) IDE Information
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