Add lightweight syntax for os.proc().call() and os.proc().spawn()

[lihaoyi]

Now these can be spelled os.call() and os.spawn(), and we provide Shellable[TupleN] conversions to make it convenient to call without constructing a Seq(...) every time. So this:

os.proc("ls", "doesnt-exist").call(cwd = wd, check = false, stderr = os.Pipe)

Becomes

os.call(cmd = ("ls", "doesnt-exist"), cwd = wd, check = false, stderr = os.Pipe)

The original purpose of the os.proc().call() style was to avoid the verbosity of constructing a Seq each time, and by making it flexible enough to take tuples, this mitigates that issue without the annoying method chaining style.

The new style still isn't shorter in terms of number of characters, but it is a lot cleaner in terms of "function call taking named/optional arguments" rather than "fluent call chain with first call taking varargs and second call taking named/optional parameters". It also aligns with the Python subprocess.* functions which OS-Lib in general is inspired by

To support Scala 2, the Shellable[TupleN] conversions are defined using codegen. Scala 3 allows a nicer generic-tuple implementation, but we'll be supporting Scala 2 for the foreseeable future.

The older os.proc.* APIs remain, both for backwards compatibility, as well as to support the pipeTo API used to construct process pipelines

Duplicated some of the existing subprocess tests to exercise the new APIs. Did not duplicate all of them, as the new APIs are pretty dumb forwarders to the existing ones so we don't need to exercise every flag in detail.

Updated the docs to point towards the new APIs, but with a mention that the older os.proc().call() style is still supported