feat: add edit input/output button

[barmac]

Proposed Changes

Run npx @bpmn-io/sr bpmn-io/dmn-js#845-edit-button-for-decision-table-head to try this out locally.

The button is visually hidden unless focussed with keyboard.

Closes #845

https://github.com/bpmn-io/dmn-js/assets/28307541/87b0de58-5779-40a1-b436-09158ea2ff25

Checklist

To ensure you provided everything we need to look at your PR:

• [x] Brief textual description of the changes present
• [x] Visual demo attached
• [x] Steps to try out present, i.e. using the @bpmn-io/sr tool
• [x] Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

@barmac Did we break the + button outline in the process?

[nikku]

The button is visually hidden unless focussed with keyboard.

Really good stuff :clap: :heart:

[barmac]

@barmac Did we break the + button outline in the process?

No, the additional outline is coming from VoiceOver.

[barmac]

So this is how it looks like without VO: