Infer plot image-save format from file extension of destination

[rolandwalker]

Description

Infer plot image-save format from file extension of destination, supporting PNG, PDF, SVG, and HTML formats.

Incidentally make the choice of "plot" over "chart" more consistent in variable names, messages, etc.

Example:

select length(table_name) as total from information_schema.tables .| df['total'].plot.hist() .> output.pdf;

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .