Use file descriptor 0 or /dev/tty to read stdin

On Linux, when Node.js (at least, v15.2.0) does a readFileSync of
/dev/tty, it hangs.

The addition of /dev/tty comes from 9bcf45b,
which converts to ESM.
It also switches from reading /dev/stdin, to:

1. on Windows, reading file descriptor 0;
2. on other platforms, reading /dev/tty
   (which presumably works on macOS, but not Linux).

However, based on the [Node.js documentation][fd0],
reading file descriptor 0 always reads stdin on all platforms,
thanks to a compatibility layer in Node.js.

Subtly, for Linux specifically, using /dev/stdin
instead of the zero file descriptor, enables the following use-case:
just enter `commonmark` on the console, causing it to wait for prompt.
You can then type text followed by Control-D
to have that text be the input.

    | File       | Linux           | macOS           | Windows   |
    |============|=================|=================|===========|
    |   |  Pipe  | ✓               | ✓               | ✓         |
    | 0 |--------|-----------------|-----------------|-----------|
    |   | Prompt | EAGAIN          | EAGAIN          | TypeError |
    |~~~~~~~~~~~~|~~~~~~~~~~~~~~~~~|~~~~~~~~~~~~~~~~~|~~~~~~~~~~~|
    |            | ✓               | ✓               | ENOENT    |
    | /dev/stdin |-----------------|-----------------|-----------|
    |            | ✓               | EAGAIN          | ENOENT    |
    |~~~~~~~~~~~~|~~~~~~~~~~~~~~~~~|~~~~~~~~~~~~~~~~~|~~~~~~~~~~~|
    |            | (input ignored) | (input ignored) | ENOENT    |
    | /dev/tty   |-----------------|-----------------|-----------|
    |            | ✓               | ✓               | ENOENT    |

Thus we split the decision in a platform-independent way:

1. When in a TTY situation, use /dev/tty.
   It does not work on Windows, but nothing simple will,
   and obviouly this is not a regression anyway.
2. Otherwise, use file descriptor 0, which works across all platforms.

[fd0]: https://nodejs.org/api/process.html#process_process_stdin_fd

Author: espadrine

bin/commonmark

@@ -3,7 +3,6 @@
 
 import util from "util";
 import fs from "fs";
-import os from "os";
 import * as commonmark from "../lib/index.js";
 var version = require('../package.json').version;
 import parseArgs from "minimist";
@@ -77,11 +76,11 @@ if (format === 'html') {
 }
 
 if (files.length === 0) {
-   if (os.platform() === "win32") {
-     inps.push(fs.readFileSync(0, 'utf-8'));
-   } else {
-     inps.push(fs.readFileSync('/dev/tty', 'utf-8'));
-   }
+  if (process.stdin.isTTY) {
+    inps.push(fs.readFileSync('/dev/tty', 'utf-8'));
+  } else {
+    inps.push(fs.readFileSync(0, 'utf-8'));
+  }
 } else {
   for (i = 0; i < files.length; i++) {
     var file = files[i];