Start a new datasette.yaml configuration file, with settings support

[asg017]

refs #2093 #2143

This is the first step to implementing the new datasette.yaml/datasette.json configuration file.

• The old --config argument is now back, and is the path to a datasette.yaml file. Acts like the --metadata flag.
• The old settings.json behavior has been removed.
• The "settings" key inside datasette.yaml defines the same --settings flags
• Values passed in --settings will over-write values in datasette.yaml

Docs for the Config file is pretty light, not much to add until we add more config to the file.

---

📚 Documentation preview 📚: https://datasette--2149.org.readthedocs.build/en/2149/

[codecov]

Codecov Report

Patch coverage: 76.47% and project coverage change: +0.10% 🎉

Comparison is base (943df09) 92.06% compared to head (db720cd) 92.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2149      +/-   ##
==========================================
+ Coverage   92.06%   92.17%   +0.10%     
==========================================
  Files          40       40              
  Lines        5937     5916      -21     
==========================================
- Hits         5466     5453      -13     
+ Misses        471      463       -8     

Files Changed	Coverage Δ
datasette/cli.py	81.02% <25.00%> (+1.20%)	⬆️
datasette/app.py	94.29% <92.30%> (-0.09%)	⬇️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[simonw]

This is a really great start - tests pass, implementation looks clean, the new stubbed documentation page makes sense. Let's land it in main and iterate on it further in future PRs.