[FIX] web_editor: differentiate images from text within Chrome iframe

Since [1] when the website UI was moved to the backend, the edited page
is inside an iframe in Chrome. Conditions based on instanceof are not
reliable anymore: the element's type is specific to the document inside
which it was created. Because of this,
`targetEl instanceof HTMLElement` is `true` when the snippet was dragged
from the options menu - because both `targetEl` and `HTMLElement` are
the ones from the main document where the editor options panel is.
But it is `false` if the page was loaded because the `targetEl` is of
a type from the iframe's document.

We cannot use `targetEl.ownerDocument.defaultView.HTMLElement` because
the `ownerDocument` is the current one to which the target belongs - not
the one where it was created. So, a full condition would have to look
like this:
```js
(targetEl instanceof this.odooEditor.document.defaultView.HTMLElement
                || targetEl instanceof HTMLElement)
```
That is pretty verbose, so I check the `nodeType` instead.

Steps to reproduce:
- Use Chrome.
- Drop a "Text - Image" snippet.
- Save.
- Edit.
- Select image.
- Press "CTRL+K".
=> An `<a class="oe_edited_link">` is created instead of triggering the
image link tools - as if it was a text selection (except the text link
tool is not available).

[1]: https://github.com/odoo/odoo/commit/31cc10b91dc7762e23b4bde9b945be0c4ce3fe3b

task-2962619

Part-of: odoo/odoo#98889

addons/web_editor/static/src/js/wysiwyg/wysiwyg.js

@@ -1080,7 +1080,7 @@ const Wysiwyg = Widget.extend({
     openLinkToolsFromSelection() {
         const targetEl = this.odooEditor.document.getSelection().getRangeAt(0).startContainer;
         // Link tool is different if the selection is an image or a text.
-        if (targetEl instanceof HTMLElement
+        if (targetEl.nodeType === Node.ELEMENT_NODE
                 && (targetEl.tagName === 'IMG' || targetEl.querySelectorAll('img').length === 1)) {
             core.bus.trigger('activate_image_link_tool');
             return;