feat: submit button callback

[silvester-pari]

Implemented changes

This PR adds a default callback for an onSubmit function, which just emits the submit event on the eox-jsonform element.

Background

json-editor supports defining a button on the form, which only is clickable once validation passes, e.g.:

button: {
  format: "button",
  options: {
    button: {
      text: "Submit",
      action: "onSubmit",
      validated: true,
    },
  },
},

In order to do something on click, an onSubmit callback must be registered before initializing the JSONEditor. In order to support a generic "submit" functionality, a onSubmit callback was registered, which dispatches a custom submit event, so that it can be listened e.g.:

<eox-jsonform
  @submit=${(e) => alert(`Submitted: ${JSON.stringify(e.detail)}`)}
></eox-jsonform>

Screenshots/Videos

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• I have added a test related to this feature/fix
• For new features: I have created a story showcasing this new feature
• All checks have passed
• The PR title follows conventional commit style and reflects the type of change (major/minor/patch, breaking)
• The PR title describes the change within this element (each merged PR equals one entry in the element's changelog!)

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