Add understrap_is_bootstrap_v4() to simplify version checking

[IanDelMar]

Description

• Addition of understrap_is_bootstrap_v4(): This new function simplifies the process of checking the Bootstrap version utilized by the theme. Instead of directly comparing the theme mod value with 'bootstrap4', developers can now use this concise and readable function, enhancing code maintainability and readability.
• Fix for $label_class in WooCommerce template: I've addressed an issue in the woocommerce/checkout/form-verify-email.php template where $label_class was incorrectly set to an empty string due to the always truthy evaluation of get_theme_mod( 'understrap_bootstrap_version', 'bootstrap4' ). This fix ensures that $label_class is accurately assigned, improving the visual consistency of the checkout form.

Motivation and Context

• Simplification and Readability: By encapsulating the Bootstrap version check within a dedicated function, we make the codebase more intuitive and easier to manage.
• Bug Fix and Consistency: Correcting the $label_class assignment ensures that the WooCommerce form remains visually consistent, enhancing the user experience.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (a code change that neither fixes a bug nor adds a feature)
• Styling (changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc))
• Development (changes that affect the build system or external dependencies)
• Documentation (changes that affect existing inline documentation)

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• (Optional) My change requires a change to the documentation.
• (Optional) I have updated the documentation accordingly.
• (Optional) My change requires a change to the translations.
• (Optional) I have updated the translations accordingly.
• composer phpcs has passed locally.
• composer php-lint has passed locally.
• composer phpmd has passed locally.
• composer phpstan has passed locally.
• I have read the CONTRIBUTING document.

Related Issues or Roadmap requests

Further comments