`fillIn` treats text as html when the target is content editable

Example:
```js
fillIn(contentEditableElement, '

');
```
The text gets parsed as html though I'd expect it to be treated as text.

Should `fillIn` set `textContent` instead of `innerHTML`?
https://github.com/emberjs/ember-test-helpers/blob/0505fdf8d581ebe78a1078db711b06cfc65e321c/addon/src/dom/fill-in.ts#L76

Start with addon/src/dom/fill-in.ts at line 76 and reproduce the issue using a contenteditable element and the provided fillIn example. Check the current handling of the supplied HTML-like text and verify that the completed behavior preserves it as text rather than parsing it as HTML.

Written by the indexing model from the issue text.