Fix RegEx for Icon library schema definition

Problem/Motivation

When trying to load a library with an icon pack it seems like the regular expression for the library property schema definition is not correct and does not allow hyphens which are valid in THEME.libraries.yml.

An IconPackConfigErrorException exception is being thrown:

Drupal\Core\Theme\Icon\Exception\IconPackConfigErrorException: my_theme:foo Error in definition `foo`:[library] Does not match the regex pattern ^\w+/[A-Za-z]+\w*$ in Drupal\Core\Theme\Icon\Plugin\IconPackManager->validateDefinition() (line 430 of core/lib/Drupal/Core/Theme/Icon/Plugin/IconPackManager.php).

Steps to reproduce

Create a new THEME.icons.yml file in your theme and try to use - in the library property:

foo:
  label: Foo
  extractor: path
  config:
    sources:
      - images/svgs/*.svg
  library: 'my_theme/an-icon-library'

Proposed resolution

The ">regular expression for validating the library should be changed from ^\w+/[A-Za-z]+\w*$ to ^[a-zA-Z0-9_]+\/[a-zA-Z0-9_-]+$ (following the RegEx from metadata.schema.json which ensures we can use hyphens in the library name after the slash.