Rewrite email ingestion driver

[gvanrossum]

Currently to ingest email (*.eml) data you run python tools/test_email.py . and enter the command @add_messages --path "gmail/email_dump".

I'd rather have a non-interactive tool similar to tools/ingest_vtt.py (e.g. tools/ingest_email.py) with a similar command line. It should have all the features of @add_messages, e.g. skipping alread-indexed messages, recovering from failures (by continuing with the next message), displaying detailed progress.

Possibly we could let it do batching like ingest_vtt, just with a default batch size of 1 (to match @add_messages).

The directory(ies) or file(s) to index should be simple command line args (not flags).

[gvanrossum]

Copilot screwed up. Will have to do this by hand or in VS Code Agent mode.