feat: support +LayerName syntax in ruleset to inherit a layer's allowed dependencies

[samsonasik]

Adds support for the +LayerName shorthand in ruleset definition.

For example, given the following config:

->ruleset([
    'API'        => ['Format'],
    'Controller' => ['Validation'],
    'RESTful'    => ['+API', '+Controller'],
])

It nows merge 2 collection of layer and its allowed dependents, which replace the manual write:

-'RESTful'    => ['API', 'Format', 'Controller', 'Validation'],
+'RESTful'    => ['+API', '+Controller'],

removing the need to duplicate those entries by hand.

Per discussion on codeigniter4/CodeIgniter4#10198 (comment)

/cc @paulbalandan @michalsn

[codecov]

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