Make TermUi internal to the library

[guuilp]

The Clikt user can be sometimes confused: should I echo or TermUi.echo? should I prompt or TermUi.prompt?

To make those answers obvious, I've made TermUi internal to the library and added the missing proxy functions: editText and editFile.

This solution also make unit testing easier, since the proxy functions on CliktCommand already passes currentContext.console to TermUil.prompt, TermUi.echo and TermUi.confirm.

[guuilp]

This is a breaking change. If you agree with this change, maybe it's worth releasing an intermediate version just adding @Deprecated to TermUi and also removing its references from the docs.

[ajalt]

Yes, we'll need to keep TermUi public for now and mark all its functions as @Deprecated with a message pointing to the new locations.

[ajalt]

Merged locally as 3371bef, thanks! I decided to leave editFile and editText as-is, since they don't use the console or other CliktCommand state.

[championswimmer]

But isn't ClicktCommand.prompt protected ? How are we to use this?