New Rule: Forbid `package_item` not under a `package_declaration`.

[DaveMcEwan]

• Noted in dalance/sv-parser#68.
• Looks generally sensible for design code.

[DaveMcEwan]

@MahmoudKMaarouf you can track progress here.

[DaveMcEwan]

@MahmoudKMaarouf If you like you can try this branch? Better late than never eh!