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

Start in src/index.ts around lines 100-102, where the markdown buttons receive their button role, and review the linked axe button-name guidance. Ensure buttons without existing accessible names have discernible text, then verify that every markdown button meets the linked criterion.

索引モデルが issue の本文から書いたものです。