Improve accessibility

[basile-parent]

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• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• I have read the documentation and cannot find an answer.

Describe the bug
If found a few easy (and yet important) improvments on accessibility:

• We cannot put id on the textarea field (div role="textbox") neither customizing its aria-label / aria-labelledby (set to aria-label="editable markdown") so we cannot customize the accessible name

• All buttons have "radio" role which has no sense if they can be activated together (for exemple the bold, italic and underline buttons are independants, they shouldn't be specified by the "radio" role). Plus, the button type="button" is redundant. For me, the only button which should be identified as radio (and grouped into a proper fieldset) are:
  • The list buttons
  • The superscript / subscript buttons

Desktop (please complete the following information):

• OS: Windows
• Browser: Brave Version 1.50.120 | Chromium: 112.0.5615.121

[basile-parent]

PR in 2 parts : #552 & #553