Use $DATASETTE_INTERNAL in absence of --internal

[asg017]

#refs 2157, specifically this comment

Passing in --internal my_internal.db over and over again can get repetitive.

This PR adds a new configurable env variable DATASETTE_INTERNAL_DB_PATH. If it's defined, then it takes place as the path to the internal database. Users can still overwrite this behavior by passing in their own --internal internal.db flag.

In draft mode for now, needs tests and documentation.

Side note: Maybe we can have a sections in the docs that lists all the "configuration environment variables" that Datasette respects? I did a quick grep and found:

• DATASETTE_LOAD_PLUGINS
• DATASETTE_SECRETS

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📚 Documentation preview 📚: https://datasette--2174.org.readthedocs.build/en/2174/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.68%. Comparing base (05707aa) to head (d75b519).
Report is 224 commits behind head on main.

Additional details and impacted files

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##             main    #2174   +/-   ##
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  Coverage   92.68%   92.68%           
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  Files          40       40           
  Lines        6012     6017    +5     
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+ Hits         5572     5577    +5     
  Misses        440      440           

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[simonw]

We can use this here: https://click.palletsprojects.com/en/8.1.x/options/#values-from-environment-variables

@click.option(..., envvar="DATASETTE_INTERNAL")

[simonw]

DATASETTE_INTERNAL would be more consistent with DATASETTE_SECRET.