fix: Run expect tests on Windows with conpty pseudo-terminal (#276)

This brings together a bunch of random, partially implemented packages for support of the new(ish) Windows [`conpty`](https://devblogs.microsoft.com/commandline/windows-command-line-introducing-the-windows-pseudo-console-conpty/) API - such that we can leverage the `expect` style of CLI tests, but in a way that works in Linux/OSX `pty`s and Windows `conpty`.

These include:
- Vendoring the `go-expect` library from Netflix w/ some tweaks to work cross-platform
- Vendoring the `pty` cross-platform implementation from [waypoint-plugin-sdk](b55c787a65/internal/pkg/pty)
- Vendoring the `conpty` Windows-specific implementation from [waypoint-plugin-sdk](b55c787a65/internal/pkg/conpty)
- Adjusting the `pty` interface to work with `go-expect` + the cross-plat version

There were several limitations with the current packages:
- `go-expect` requires the same `os.File` (TTY) for input / output, but `conhost` requires separate file handles
- `conpty` does not handle input, only output
- The cross-platform `pty` didn't expose the full set of primitives needed for `console`

Therefore, the following changes were made:
- Handling of `stdin` was added to the `conpty` interface
- We weren't using the full extent of the `go-expect` interface, so some portions were removed (ie, exec'ing a process) to simplify our implementation and make it easier to extend cross-platform
- Instead of `console` exposing just a `Tty`, it exposes an `InTty` and `OutTty`, to help encapsulate the difference on Windows (on Linux, these point to the same pipe)

Future improvements:
- The `isatty` implementation doesn't support accurate detection of `conhost` pty's without an associated process. In lieu of a more robust check, I've added a `--force-tty` flag intended for test case use - that forces the CLI to run in tty mode.
- It seems the windows implementation doesn't support setting a deadline. This is needed for the expect.Timeout API, but isn't used by us yet.

Fixes #241

cli/login.go

@@ -22,6 +22,7 @@ func login() *cobra.Command {
 		Args: cobra.ExactArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			rawURL := args[0]
+
 			if !strings.HasPrefix(rawURL, "http://") && !strings.HasPrefix(rawURL, "https://") {
 				scheme := "https"
 				if strings.HasPrefix(rawURL, "localhost") {
@@ -44,7 +45,7 @@ func login() *cobra.Command {
 				return xerrors.Errorf("has initial user: %w", err)
 			}
 			if !hasInitialUser {
-				if !isTTY(cmd.InOrStdin()) {
+				if !isTTY(cmd) {
 					return xerrors.New("the initial user cannot be created in non-interactive mode. use the API")
 				}
 				_, _ = fmt.Fprintf(cmd.OutOrStdout(), "%s Your Coder deployment hasn't been set up!\n", color.HiBlackString(">"))