This commit adds support for rustc's `-Z terminal-width` flag, which is
used to trim diagnostic output to fit within the current terminal.

Co-authored-by: David Wood <david@davidtw.co>
Signed-off-by: David Wood <david@davidtw.co>

This commit modifies the parsing of `-Z terminal-width` so that it can
optionally take a value and only uses `accurate_err_width` when the user
does not provide a value - therefore making the emission of `-Z
terminal-width` opt-in.

Signed-off-by: David Wood <david@davidtw.co>

This commit adds relevant documentation for the `-Z terminal-width`
flag.

Signed-off-by: David Wood <david@davidtw.co>

This commit introduces a `TtyWidth` enum which enables better handling
of the tty-width on Windows.

Signed-off-by: David Wood <david@davidtw.co>