New CSS component: Input

[devmount]

Is your component request related to a problem? Please describe.
Form inputs are already supported by core styles. However, it would be nice to have some additional styling for further use cases.

Describe the component you'd like for that problem

• Providing different sizes
• Providing help text for inputs
• Providing prefix and suffix icons for inputs
• Providing prefix and suffix labels for inputs
• Providing a busy state for an input element, containing an indicator, e.g. with a .busy class
• Introducing a new class .input-group for the parent element, to make multiple inputs appear as a single, segmented input.
• Making inputs work with utility class rounded-full rounded e.g. with a .pill class
• Providing ghost inputs without styling