WIP: create sqlx.toml format

[abonander]

Reference sqlx.toml: https://github.com/launchbadge/sqlx/blob/sqlx-toml/sqlx-core/src/config/reference.toml

Once this is implemented, I'm planning on starting an alpha release cycle for 0.9.0 so that we can test this. It's not expected to be a breaking change but it's a large enough feature that it should probably go in a new major release, and there's a few other breaking API changes waiting in the wings.

The keys listed here are just the ones I've identified as needed for an MVP. Other keys may be added later backwards-compatibly.

Bikeshedding checklist:

• snake_case or kebab-case for keys?
• macros.column_overrides should maybe be called macros.table_overrides instead?
  • since the next level in the key is the table name, not the column name?

Implementation checklist

• [common]
  • database-url-var
    • support in query macros
    • support in sqlx-cli
    • test
• [macros]
  • preferred-crates
    • date-time
      • test
    • numeric
      • test
  • table-overrides
    • test
  • type-overrides
    • test
• [migrate]
  • table-name
    • support in migrate!()
    • support in sqlx-cli
    • test
  • migrations-dir
    • support in migrate!()
    • support in #[sqlx::test]?
    • support in sqlx-cli
    • test
  • ignored-chars
    • support in migrate!()
    • support in sqlx-cli
    • test
  • defaults
    • migration-type
      • test
    • migration-versioning
      • test

closes #121
closes #916
closes #3412