Add support for canned query parameters with defaults and longer descriptions. (Fixes #1258)

[etjones]

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📚 Documentation preview 📚: https://datasette--2489.org.readthedocs.build/en/2489/

[etjones]

This is in response to #1258.

Canned queries have had this format:

databases:
  fixtures:
    queries:
      neighborhood_search:
        params:
        - text

That's still the default, but this PR enables queries to have default values and tooltip descriptions as well:

params:
- name: text
  default: ""
  description: "Longer description of the kind of text you want"