Use terminal width for top-level constraints

[mattprecious]

Unless something in the tree applies a width modifier or adjusts constraints in a custom Layout, the maxWidth will default to Infinity.

I think it would be a quality-of-life improvement to default this to the terminal width if available. Rendering beyond the terminal width will usually result in an unintentional wrap which breaks mosaic's render/reset loop.

Note: Text will currently throw an exception if measured at a width smaller than its intrinsic width, so I don't know if this change can be made without also doing something about text overflow.

[JakeWharton]

Constrain the width to terminal size, but keep infinite height.

Alternate screen (#455) will constrain both width and height to terminal size.

[JakeWharton]

For anyone watching, lots of problems encountered when attempting this due to #767