feat(components): add checkbox group component

[desmondinho]

Description

Add the checkbox group component, a built-in wrapper for checkboxes, to the component library.

Closes #22

What's been done?

Added the checkbox group component with the following abilities:

• Provide a label
• Provide a description
• Set the disabled state (globally)
• Set the read-only state (globally)
• Set the theme color (globally, e.g., primary, danger)
• Set the border radius (globally, e.g., none, medium)
• Set the size (globally, e.g., small, large)
• Set CSS class names for each slot
• Set CSS class names for each checkbox slot

.. and updated the checkbox component to be aware of its potential context within a checkbox group, ensuring it prioritizes its own parameter values (e.g., color, size, radius) over those of the group.

Checklist

• [x] My code follows the project's coding style and guidelines.
• [x] I have included inline docs for my changes, where applicable.
• [x] I have added, updated or removed tests according to my changes.
• [x] All tests are passing.
• [x] There's an open issue for the PR that I am making.