Translate `if`/`then`/`else` to `oneOf`

Right now we don't support the if/then/else structure. I think it would be valid to transform it to a oneOf:

{
  "if": { "$ref": "#/$defs/A" },
  "then": { "$ref": "#/$defs/B" },
  "else": { "$ref": "#/$defs/C" },
}

transforms to

{
  "oneOf": [
    {
      "title": "then",
      "allOf": [
        { "$ref": "#/$defs/A" },
        { "$ref": "#/$defs/B" }
      ]
    },
    {
      "title": "else",
      "allOf": [
        { "not": { "$ref": "#/$defs/A" } },
        { "$ref": "#/$defs/C" }
      ]
    }
  ]
}

This should become something like:

#[serde(untagged)]
pub enum IfThenElseType {
    Then(BA),
    Else(CNotA),
}

The issue names no files or tests. Start by tracing existing oneOf handling in the JSON Schema-to-Rust compiler, then check whether the proposed if/then/else transformation preserves the conditional schema semantics. Done means supporting the structure and generating the corresponding untagged Rust enum representation.