jsondoclint: Add option to dump errors as json.

The output looks like:
{
  "errors": [
    {
      "id": "2:2017:1833",
      "kind": {
        "NotFound": [
          [
            {"Field": "index"},
            {"Field": "0:0:1571"},
            {"Field": "links"},
            {"Field": "pointer::read"}
          ]
        ]
      }
    }
  ],
  "path": "/home/nixon/dev/rust/rust/build/x86_64-unknown-linux-gnu/test/rustdoc-json/intra-doc-links/pointer_method/pointer_method.json"
}
diff --git a/Cargo.lock b/Cargo.lock
index 0483d50..bb438f2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2115,6 +2115,7 @@
  "clap 4.0.15",
  "fs-err",
  "rustdoc-json-types",
+ "serde",
  "serde_json",
 ]