feat: add OrderedMap, the table that keeps its key order

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -46,6 +46,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   (10000 levels, which no hand-written document approaches): a document that
   nests arrays or inline tables deeper used to run the stack out and is now
   rejected with a `SyntaxError` naming the limit.
+- `OrderedMap`, the string-keyed table that remembers the order its keys were
+  set in: decoding into one fills it in the order the document wrote the
+  keys, and `Marshal` writes one back in that order, where a map carries no
+  order on decode and sorts on encode. It works as a decode target on its
+  own, in a struct field, and as the element of an array of tables; its
+  values are untyped, so a nested table stays a `map[string]any`.
 - The `toml` tag gained the `required` option: a field tagged
   `toml:"host,required"` makes the decode fail with
   `missing required key "host"` when the document carries no key that