Add Plugin_Header_Requires_Check

[ernilambar]

Fixes https://github.com/WordPress/plugin-check/issues/405

• Add Plugin_Header_Requires_Check
• Tests added for Plugin_Header_Requires_Check
• For single file plugin or plugin without readme file, headers of main plugin is checked. ERROR is raised if no headers found
• For multi file plugin, ERROR is raised if headers are missing from both main plugin file or readme file

[ernilambar]

cc: @swissspidy @mukeshpanchal27 @felixarntz

[swissspidy]

Let's discuss on the ticket first.

PS Felix is on leave right now, best not to ping him :)

[ernilambar]

Closing this for now. We can reopen in the future if needed.