feat: add the Document edit pipeline with comment-preserving write

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -32,8 +32,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   values together with the key order, whether a table was written as an inline
   table or under a header, and the comments, with `Keys`, `Entries`, `Get`,
   `Comments` and `SetComments` to read and write them. `ParseMap` returns the
-  plain `map[string]any` tree, the shape `Parse` used to give. `Marshal` does
-  not accept a `Document`; it writes values, so `doc.Map()` is the way through.
+  plain `map[string]any` tree, the shape `Parse` used to give.
+- The edit pipeline on a `Document`: typed getters on `Document` and `Table`
+  (`GetString`, `GetInt`, `GetFloat`, `GetBool`, `GetArray`, `GetTable`),
+  `Set` and `Delete` that keep the surviving keys' positions and comments,
+  `UnmarshalDocument`, which decodes the document into a typed destination
+  without parsing again, and `Marshal` of a `Document`, which writes the keys
+  in written order, the comments above the lines and headers they belonged
+  to, and the inline tables inline.
 - `OffsetDateTime`, the Go type of the offset date-time kind, so that all four
   TOML date-time kinds have one of their own. `Parse` and `Unmarshal` hand it
   back where they produced a bare `time.Time` before, and `Marshal` accepts it.