Append submitter entry in submissions

[PaperStrike]

Motivation and Context

In form submissions, the form entry list should include the submitter's entry, if the submitter is not disabled and has a non-empty name. See the algorithm to construct the entry list, which the form submission algorithm uses.

There isn't a straightforward API to construct the entry list of a form given a submitter, yet. Track the possible future solution here in whatwg/xhr#262.

How Has This Been Tested?

Unfortunately, there isn't a plan for Submission lib's test cases, yet.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no bug fix and new feature but improvements)

Checklist:

• My change requires new tests.
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.