Add Wiki format tracking

What does this MR do and why?

A very small MVC for: https://gitlab.com/gitlab-org/gitlab/-/issues/335656 where we add basic client sidetracking for the Wiki format based on Wiki form submissions.

When a form is submitted we now log the format, project_path, and old_format.

{
  label: WIKI_FORMAT_LABEL, // 'wiki_format'
  value: this.format, // 'markdown'
  extra: { 
    project_path: this.pageInfo.path, // 'project/path/wiki'
    old_format: this.pageInfo.format // 'markdown'
  },
}

N/A

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #335656