cherry-pick of fbb364c1cd0dec9e4a75b885d51e67ca397b1d6a:
Fixes focus and blur management when `page.locator(…).focus()` and `page.locator(…).type(…)` are used which was regressed by 7a5b070 (#13510).
However, some elements are [not focusable](https://html.spec.whatwg.org/multipage/interaction.html#focusable-area), so we were blurring incorrectly, and losing focus that we should have maintained.
Two regression tests were added that pass on the commit prior to 7a5b070e95 (and match manual testing/expectations):
* `page.locator(…).focus()`: _keeps focus on element when attempting to focus a non-focusable element_
* `page.locator(…).type(…)`: _should type repeatedly in input in shadow dom_
Additionally, a third test (_should type repeatedly in input in shadow dom_) was added to check the invariant from #13510 that states:
> This affects [contenteditable] elements, but not input elements.
and allows us to introduce the targeted fix (contenteditble check before blur) without breaking FF again.
And _should type repeatedly in contenteditable in shadow dom with nested elements_ was added to ensure the above fix works with nest contenteditble detection.
Fixes#14254.
abd4258080