fix(dfn): fix schema version migrations

[wpbonelli]

Backwards-compatibility fixes, schema version naming/differentiation, and usability improvements.

• Add new migrate subcommand to DFNs CLI
• Disentangle migration logic from object model
• As we have prototyped the new schema, the code to generate/represent intermediate versions has grown messy. We need to clearly distinguish schema versions, especially since flopy3 codegen consumes one of them. Identify the following versions
  • 1: original DFNs, flat set of components, each a flat list of fields plus comment lines
  • 2.0.0.dev0: flat set of components, some structuring of fields, consumed by flopy3
  • 2.0.0.dev1: tree of components, more extensive field structuring, consumed by flopy4
  • 2.0.0.dev2: pydantic redesign, flat set of components, structured blocks/fields

Closes #259. The modflow_devtools.dfn module stays stable and supports only the 2.0.0.dev0 schema version expected by flopy3. The new dfns module gets some improvements and supports all three prototype schema versions. This versioning schema can continue until we release version 2.