Redesign row default JSON

This URL here:

https://latest.datasette.io/fixtures/simple_primary_key/1.json?_extras=foreign_key_tables

{
  "database": "fixtures",
  "table": "simple_primary_key",
  "rows": [
    {
      "id": "1",
      "content": "hello"
    }
  ],
  "columns": [
    "id",
    "content"
  ],
  "primary_keys": [
    "id"
  ],
  "primary_key_values": [
    "1"
  ],
  "units": {},
  "foreign_key_tables": [
    {
      "other_table": "foreign_key_references",
      "column": "id",
      "other_column": "foreign_key_with_blank_label",
      "count": 0,
      "link": "/fixtures/foreign_key_references?foreign_key_with_blank_label=1"
    },
    {
      "other_table": "foreign_key_references",
      "column": "id",
      "other_column": "foreign_key_with_label",
      "count": 1,
      "link": "/fixtures/foreign_key_references?foreign_key_with_label=1"
    },
    {
      "other_table": "complex_foreign_keys",
      "column": "id",
      "other_column": "f3",
      "count": 1,
      "link": "/fixtures/complex_foreign_keys?f3=1"
    },
    {
      "other_table": "complex_foreign_keys",
      "column": "id",
      "other_column": "f2",
      "count": 0,
      "link": "/fixtures/complex_foreign_keys?f2=1"
    },
    {
      "other_table": "complex_foreign_keys",
      "column": "id",
      "other_column": "f1",
      "count": 1,
      "link": "/fixtures/complex_foreign_keys?f1=1"
    }
  ],
  "query_ms": 4.226590999678592,
  "source": "tests/fixtures.py",
  "source_url": "https://github.com/simonw/datasette/blob/main/tests/fixtures.py",
  "license": "Apache License 2.0",
  "license_url": "https://github.com/simonw/datasette/blob/main/LICENSE",
  "ok": true,
  "truncated": false
}

That ?_extras= should be ?_extra= - plus the row JSON should be redesigned to fit the new default JSON representation.

Start by reproducing the row JSON response at the URL in the issue and inspect the fixture source mentioned there, tests/fixtures.py. Confirm the _extra spelling and determine the intended new default row representation before identifying the affected endpoint tests; done means the response uses the corrected parameter and the agreed representation.