perf: pre-declare position on delete factory

Problem: the strikethrough handler creates each delete node as a
fresh object literal, then mdast-util-from-markdown's enter() sets
node.position afterwards. V8 treats that late assignment as a shape
change, so every delete node walks through two hidden classes instead
of one. Core nodes already avoid this by declaring position up front;
without the same trick on extension nodes, GFM trees end up mixing
two layouts and downstream tree walks lose monomorphic dispatch.

Goal: declare position on the delete literal at construction so V8
keeps a single hidden class for delete from start to finish.

Changes:
- lib/index.js: trailing position: undefined on the delete literal
  in enterStrikethrough.

Notes: pairs with the matching core change in
syntax-tree/mdast-util-from-markdown#53;
merged on its own this branch is a no-op. Validated with npm test
(build, format, 100% coverage, 13 of 13 tests in dev and prod).

Bench (local harness, 3-run median-of-medians, GFM tokenizer and
mdast extensions stacked, with the core branch and three sibling
extension branches checked out alongside this one):

Input: one paragraph of 1000 ~~strike~~ runs. On the baseline this
is the second-largest mdast-layer share in the GFM corpus (full
11.0 ms, tokenize-only 5.2 ms; the mdast layer is the gap between
those two).

  setup                          delta vs main
  core branch alone              +14.6%
  core branch with this paired   +2.6%

The 12-point swing is the recovery this branch is designed to produce:
delete nodes now share the core layout, so tree walks stay monomorphic.
A drift floor measured separately (main against itself, no code
change) ran at +4 to +12 percent across most GFM inputs on this
machine in the same window, so the +2.6 percent residual sits inside
bench noise.

Author: ChristianMurphy

lib/index.js

@@ -71,7 +71,10 @@ export function gfmStrikethroughToMarkdown() {
  * @type {FromMarkdownHandle}
  */
 function enterStrikethrough(token) {
-  this.enter({type: 'delete', children: []}, token)
+  // Trailing `position: undefined` keeps the delete hidden class stable;
+  // mdast-util-from-markdown's enter() patches the field to a real value,
+  // but the property already exists, so no shape transition fires.
+  this.enter({type: 'delete', children: [], position: undefined}, token)
 }
 
 /**