New sniff "NamingConventions.ValidNavMenuSlug"

Is your feature request related to a problem?

As of WP 5.3, integer menu location slugs will no longer be supported.

Refs:

• https://make.wordpress.org/core/2019/09/18/integer-menu-slugs-are-no-longer-supported-from-wordpress-5-3/
• https://core.trac.wordpress.org/ticket/45361
• https://developer.wordpress.org/reference/functions/register_nav_menus/
• https://developer.wordpress.org/reference/functions/register_nav_menu/

Describe the solution you'd like

I'd like to suggest we add a sniff to the NamingConventions category to check code for this.

While this sniff could be a candidate for the Core ruleset, at the very least, it should go into the Extra ruleset (and the WPThemeReview ruleset /cc @dingo-d ).

// OK.
register_nav_menus(
    array(
        'primary' => 'Primary', 
        'first' => 'First', 
        'se' . 'cond' => 'Second',
    )
);

// Ignore.
register_nav_menus( $locations );
register_nav_menus( $obj->get_locations() );

register_nav_menus(
    array(
        MENU_ONE => 'First', 
        $second => 'Second',
    )
);

// Bad.
register_nav_menus(
    array(
        1 => 'First',
        'Second',
        2.5 => 'Third',
        false => 'Fourth',
    )
);

To find inspiration for additional unit tests: https://wpdirectory.net/search/01DN3RPKFHVQ712NG72GP99D5D

Additional context (optional)

Considerations to keep in mind when writing the sniff:

• If $locations is passed as a variable, constant or via a function call, the sniff should just bow out as it will be very difficult to figure out the value in a reliable manner.
• The sniff should check for both array values without key, as well as array values with a non-string key.
  Keys set via a variable or constant should be ignored.
• The register_nav_menu() function should be checked as well, as, while the $location parameter is documented as a string, the function does no variable type checking.

To be researched

Are there any other restrictions to menu slugs ? And if so, should those be covered in the same sniff ?

Think along the lines of:

• Names which shouldn't be used (reserved).
• Any restrictions to the slug name ? allowed characters, slug length, etc

Input welcome!

Start with the register_nav_menus() and register_nav_menu() examples and the linked WordPress references, then review the proposed NamingConventions category and Extra, Core, and WPThemeReview rulesets. Define the supported menu-slug restrictions and test cases for literal arrays, before considering implementation complete.