Flesh out, polish and document REPL.Terminals

[KronosTheLate]

I have found some of the functionality in REPL.Terminals very useful, namely the functionality to move the cursor.

However, it is lacking in documentation, and seems only partly finished.
I would love to see it polished up more, with added docstrings and example usage. Otherwise, it feels like it should be deprecated, or be more limited in scope.

For example, the following code is found there

# Terminal interface:
pipe_reader(::TextTerminal) = error("Unimplemented")
pipe_writer(::TextTerminal) = error("Unimplemented")
displaysize(::TextTerminal) = error("Unimplemented")
cmove(t::TextTerminal, x, y) = error("Unimplemented")
getX(t::TextTerminal) = error("Unimplemented")
getY(t::TextTerminal) = error("Unimplemented")
pos(t::TextTerminal) = (getX(t), getY(t))

But it seems like there is no version of these functions other than these fallbacks. Furthermore, there is a type hierarchy with AbstractTerminal, TextTerminal, and UnixTerminal, but only UnixTerminal exists as a concrete type. This seems very partly finished. So the scaffolding should be fleshed out, or removed.

[KronosTheLate]

#50154 is a start on this, by adding default TextTerminal in function calls, greatly enhancing usability.

[KronosTheLate]

In general, I would love to see the functions for cursor movement use print and not write, as the returned integer is generally just noise. That would however be breaking of exported functions, which is no good. Perhaps that should be added to a Julia 2.0 milestone, if we can agree that the returned integer is just noise in almost all use-cases?