fix(xml): use the type system to verify version

Reverts: 78ed4ac

xml/_types.ts

@@ -25,11 +25,11 @@ export type xml_node = {
 /** XML document. */
 export type xml_document = xml_node & {
   /** XML version. */
-  ["@version"]?: string
+  ["@version"]?: `1.${number}`
   /** XML character encoding. */
   ["@encoding"]?: string
   /** XML standalone. */
-  ["@standalone"]?: string
+  ["@standalone"]?: "yes" | "no"
   /** XML doctype. */
   ["#doctype"]?: xml_node
   /** XML instructions. */

xml/_types_test.ts

@@ -1 +1,18 @@
 import "./_types.ts"
+import { stringify } from "./mod.ts"
+
+// Verify that the generic type is correctly inferred.
+const _example = stringify({
+  "@version": "1.1",
+  "@encoding": "UTF-8",
+  "@standalone": "yes",
+  "#doctype": {
+    "~name": "html",
+    "~children": [],
+    "#text": "",
+  },
+  "#instructions": {},
+  "~name": "html",
+  "~children": [],
+  "#text": "",
+})