Buttons must have discernible text

Just as we set the `role=button` for the markdown buttons:

https://github.com/github/markdown-toolbar-element/blob/731b52a7e400260a693ff745ddf3e5984a42b411/src/index.ts#L100-L102

We should also give them [discernible text][1] if it's not already set.

[1]: https://dequeuniversity.com/rules/axe/4.0/button-name?application=axeAPI