…a` to `Core`

> 1. When using multiple modifiers for a property or method declaration, the order should be as follows:
>     1. First the optional `abstract` or `final` keyword.
>     2. Next a visibility keyword.
>     3. Lastly, the optional `static` [red: or `readonly`] keyword.

For property declarations, this is already covered by the `PSR2.Classes.PropertyDeclaration` sniff.
For method declarations, the `PSR2.Methods.MethodDeclaration` sniff has now been moved from `Extra` to `Core`.

The `PSR2.Methods.MethodDeclaration` sniff contains one additional error code, which IMO should probably not (yet) go into the `Core` ruleset.
* `Underscore` - which warns against prefixing a method name with an underscore, advising to use visibility instead.
    While I'm 100% in favour of this, introducing this for WP Core would be problematic as renaming (`public`/`protected`) methods would be a BC-break.

As this rule was previously already included in the `Extra` ruleset, the error code which is being silenced for `Core`, will be "unsilenced" again for `Extra`.

The `PSR2.Classes.PropertyDeclaration` sniff does not yet account for ordering the visibility vs readonly keywords.
I have opened a PR upstream - squizlabs/PHP_CodeSniffer 3637 - proposing to add a check for this to the `PSR2.Classes.PropertyDeclaration` sniff.
* If that PR would be accepted, WPCS will automatically inherit the check and we'd be good.
* If that PR would be rejected, I propose to create a dedicated sniff in PHPCSExtra to handle property modifier keywords with configuration options for the preferred order.

Refs:
* https://make.wordpress.org/core/2020/03/20/updating-the-coding-standards-for-modern-php/ - Visibility should always be declared section
* https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#visibility-and-modifier-order
* WordPress/wpcs-docs 108
* WordPress/WordPress-Coding-Standards 1101
* WordPress/WordPress-Coding-Standards 1103
* squizlabs/PHP_CodeSniffer 3637

While this was not explicitly mentioned in the Make post, it is implied that there should be exactly one space after each modifier keyword for properties and methods.

This was already being checked for in `Extra`. I'm proposing to move this sniff to the `Core` ruleset.

Note: I've checked and introducing this sniff to the `Core` ruleset does not yield any issues for WP Core as is, so it would be a silent introduction.

Refs:
* WordPress/WordPress-Coding-Standards 1101