Fix template element parsing and innerHTML handling

- Fix template children being parsed into element.childNodes instead of template.content during parseHTML()
- Add special handling in parse-from-string.js to redirect template children to content DocumentFragment
- Implement proper template.innerHTML getter/setter to access content instead of direct children
- Add template.toString() method following TextElement pattern to include content in serialization
- Ensure template element follows HTML spec where innerHTML always references template.content
- Add null safety checks in parsing logic to prevent errors during template processing

The template element now correctly:
- Parses children into content during HTML parsing
- Returns content innerHTML when accessing template.innerHTML
- Maintains separation between direct children and content
- Serializes with content included in toString()

--browser result--
const f = document.createDocumentFragment();
const d = document.createElement('div')
d.innerHTML='<a>aaa</a>';
f.append(d);
const t = document.createElement('template');
t.content.append(f);
console.log(t.innerHTML); // '<div><a>aaa</a></div>'
console.log(t.innerText); //''

--linkedom--
console.log(t.innerHTML); // ''
console.log(t.innerText); //''

Author: visualkhh

cjs/shared/parse-from-string.js

@@ -111,7 +111,6 @@ const parseFromString = (document, isHTML, markupLanguage) => {
     onclosetag() {
       if (isHTML && node === ownerSVGElement)
         ownerSVGElement = null;
-      // template content인 경우 원래 template로 돌아가기
       if (node && node[PRIVATE]) {
         node = node[PRIVATE];
       }

esm/shared/parse-from-string.js

@@ -109,7 +109,6 @@ export const parseFromString = (document, isHTML, markupLanguage) => {
     onclosetag() {
       if (isHTML && node === ownerSVGElement)
         ownerSVGElement = null;
-      // template content인 경우 원래 template로 돌아가기
       if (node && node[PRIVATE]) {
         node = node[PRIVATE];
       }