Markdown editor toolbar buttons defaults to tabIndex=-1

**Is your feature request related to a problem? Please describe.**

Due an accessibility report, we've saw that the current version doesn't support change tabindex="-1" to "0". Currently behaviour removes removes the element from the navigation sequence.

**Describe the solution you'd like**

Wondering if would be possible to expose the tabindex attribute programmatically. Also want to understand the reasons to fix that in the code.

**Describe alternatives you've considered**

We can make it manually focusable using javascript, but would be easier just setting the tabindex. Thanks in advance for the review.

Reference in code:

```
el.tabIndex = -1;
```