Test and document advanced form attributes

[daun]

Description

• I've been needing this for a project, but wasn't sure if the plugin supported these
• So I figured why not create the tests and documentation for everybody else who might wonder
• Add tests for externally associated form elements
• Add documentation on supported form attributes
  • Externally associated form elements via form attribute
  • Form submitter overrides for action, target, enctype and target

Checks

• The PR is submitted to the main branch
• The code was linted before pushing (npm run lint)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

[hirasso]

Testing the web platform ... nice 😄

...I didn't even know these existed. Now I know.