Resolves https://github.com/pyodide/pyodide/issues/3112
This adds a carefully designed API for controlling stdin, stdout, and stderr. It changes
the default behavior to be a bit more useful, though in doing so introduces some mild
backwards incompatibility. In particular:
1. By default, stdin reads directly from `process.stdin` in node (as before) and raises an
error if in browser (not as before).
2. By default, stdout writes directly to `process.stdout` in node (before it called console.log)
and calls console.log in browser (as before).
3. By default, stderr writes directly to `process.stderr` in node (before it called console.warn)
and calls console.warn in browser (as before).
4. In all three cases, by default isatty(stdin/stdout/stderr) is true in node and false in browser
(in the browser it used to be true).
5. As before, if you pass `stdin`, `stdout`, or `stderr` as arguments to `loadPyodide`, `isatty` of
the corresponding stream is set to false.
The stdin function is now more flexible: we now correctly handle the case where it returns an
ArrayBuffer or ArrayBufferView.
I also added 3 new functions to set streams after Pyodide is loaded which offer additional
control:
* `setStdin({stdin?, error?, isatty = false})` -- Sets the stdin function. The stdin function takes no
arguments and should return null, undefined, a string, or a buffer. Sets and `isatty(stdin)` to
`isatty` (by default `false`). If error is true, set stdin to always raise an EIO error when it is read.
* `setStdout({raw?, batched?, isatty = false})` -- If neither raw nor batched is passed, restore
default stdout behavior. If rwa is passed, the raw stdout function receives a byte which it should
interpret as a utf8 character code. Sets `isatty(stdout)` to isatty (by default `false`). If batched is
passed but not raw, it sets a batched stdout function. The stdout function receives a string and
should do something with it. In this case it ignores isatty and sets isatty(stdout) to false.
* `setStderr({raw?, batched?, isatty = false})` -- same but with stderr.
Hood Chatham