[6.x] Input only shows copy button if supported

[jasonvarga]

When you're working locally without https, the browser clipboard api doesn't work. The copy button would do nothing.
See #12074

This PR will hide the button in that case.

This PR also does some necessary clean up to Input component.

• hasPrependedIcon and hasAppendedIcon are computeds which allows it to adapt to prop changes (and so we can use the copyable computed in it)
• iconClasses is a computed (like inputClasses already was) and performs twMerge. The large diff there is really due to indentation.
• inputClasses and iconClasses use .value because they now reference computeds
• Removed unnecessary .value usage from the template