[6.x] Append configs before and after

[jackmcdade]

This allows us to do this, and append Automagic Forms settings as the second section:

private function bootAutomagicForms(): void
{
    Form::appendConfigFields(
        handles: '*',
        display: 'Automagic Forms',
        fields: [
          // ...
        ],
        afterSection: 'name'
    );
}

• beforeSection / afterSection take a section handle (name, fields, submissions, email). Only one may be passed.
• Positions only apply when a new section is created. If display matches an existing section, the fields are merged into it, and passing a position throws an InvalidArgumentException.
• If the target section doesn't exist (e.g. a typo, or an addon section registered later), the section is appended at the end and a warning is logged.
• If a positioned section's handle collides with an existing section, it replaces that section in place (as unpositioned registrations already do) and a warning is logged that the position was ignored.
• Positions aren't included in Form::extraConfigFor() (which also feeds form_config in the form tag and emails). They're available via Form::extraConfigPositionsFor().