We use a third-party library "colorable" to translate VT100 color
sequences into Windows console attribute-setting calls when Terraform is
running on Windows.
colorable is not concurrency-safe for multiple writes to the same console,
because it writes to the console one character at a time and so two
concurrent writers get their characters interleaved, creating unreadable
garble.
Here we wrap around it a synchronization mechanism to ensure that there
can be only one Write call outstanding across both stderr and stdout,
mimicking the usual behavior we expect (when stderr/stdout are a normal
file handle) of each Write being completed atomically.
