This is a helper package that creates a very thin abstraction over
terminal setup, with the main goal being to deal with all of the extra
setup we need to do in order to get a UTF-8-supporting virtual terminal
on a Windows system.

We need to call into terminal.Init in early startup to make sure that we
either have a suitable Terminal or that we disable attempts to use virtual
terminal escape sequences.

This commit gets the terminal initialized but doesn't do much with it
after that. Subsequent commits will make more use of this.

Here we propagate in the initialized terminal.Streams from package main,
and then onwards to backends running in CLI mode.

This also replaces our use of helper/wrappedstreams to determine whether
stdin is a terminal or a pipe. helper/wrappedstreams returns incorrect
file descriptors on Windows, causing StdinPiped to always return false on
that platform and thus causing one of the odd behaviors discussed in

Finally, this includes some wrappers around the ability to look up the
number of columns in the terminal in preparation for use elsewhere. These
wrappers deal with the fact that our unit tests typically won't populate
meta.Streams.

We now require the output to accept UTF-8 and we can determine how wide
the terminal (if any) is, so here we begin to make use of that for the
"terraform plan" command.

The horizontal rule is now made of box drawing characters instead of
hyphens and fills the whole terminal width.

The paragraphs of text in the output are now also wrapped to fill the
terminal width, instead of the hard-wrapping we did before.

This is just a start down the road of making better use of the terminal
capabilities. Lots of other commands could benefit from updates like these
too.

We were previously using some ASCII art to create some visual divisions
between parts of the diagnostic output. Now that we are requiring a UTF-8
terminal we can print out box drawing characters instead.

In some terminal emulators, writing a character into the last column on a
row causes the terminal to immediately wrap to the beginning of the next
line, even if the very next character in the stream is a hard newline.
That can then lead to errant blank lines in the final output which make
it harder to navigate the visual hierarchy.

As a compromise to avoid this, we'll format our horizontal rules and
paragraphs to one column less than the terminal width. That does mean that
our horizontal rules won't _quite_ cover the whole terminal width, but
it seems like a good compromise in order to get consistent behavior across
a wider variety of terminal implementations.