Expand what #config_target can do: automatically set #attributes[pattern] for HTML5 client-side validation when RegEx constraint is present

Problem/Motivation

#3398982: ConfigFormBase + validation constraints: support non-1:1 form element-to-config property mapping again introduced the necessary infrastructure to remove validation logic from forms, move them into config schema instead, and have the validation constraints in config schema automatically applied.

A few days ago #3395555: Add validation constraints to olivero.settings landed, which removed olivero_theme_settings_validate() because it now is obsolete, thanks to Olivero's config schema defining validation constraints and its theme settings form having adopted #config_target🚀.

Which also reveals an opportunity for building on top of what #3398982 did and removing the need for duplication. This change landed:

-      '#default_value' => theme_get_setting($key),
+      '#config_target' => "olivero.settings:$key",
      '#attributes' => [
        // Regex copied from Color::validateHex()
        'pattern' => '^[#]?([0-9a-fA-F]{3}){1,2}$',

— the commit for #3395555.

But note how [#attributes' => ['pattern' => …]] still repeats the exact same regex. This is what enables HTML 5's native client-side validation.

Steps to reproduce

N/A

Proposed resolution

Automatically set [#attributes' => ['pattern' => …]].

Remaining tasks

Implement this.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

HTML 5 client-side validation is now automatically applied to config forms for those form elements whose corresponding config key has the RegEx validation constraint defined in its config schema.