Buttons must have discernible text

[koddsson]

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

markdown-toolbar-element/src/index.ts

Lines 100 to 102 in 731b52a

	if (!this.hasAttribute('role')) {
	this.setAttribute('role', 'button')
	}

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