Converts inline panel anchor to button to make keyboard focusable.

[jessemenn]

This allows the InlinePanel button to be keyboard navigable and targetable with VoiceOver.

Before submitting, please review the contributor guidelines https://docs.wagtail.io/en/latest/contributing/index.html and check the following:

• For front-end changes: Did you test on all of Wagtail’s supported environments?
  • Please list the exact browser and operating system versions you tested.
    • macOS Chrome, Firefox ESR, Safari
  • Please list which assistive technologies you tested.
    • VoiceOver on macOS with Safari

[squash-labs]

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Test this branch here: https://jessemenninline-panel-button-d50u0.squash.io

[Stormheg]

Thanks @jessemenn 🚀 ⌨️

This looks good and I can confirm this solves the issue.

Here are some things I noticed during testing with bakery demo, out of scope for this PR

• 'ADD' is spelled instead of pronounced as a word. I think this is a common issue in Wagtail in general.
• There is no feedback after clicking the button. I would expect focus to change to the new item and VoiceOver to announce a new item has been added.