Refactor PTY

PTY now provides a public interface to an underlying instance of
PTYNative. The PTYNative class can be extended as appropriate for
each platform and the platform-specific implementation will be
selected by PTY at runtime.

addons/godot_xterm/nodes/pty/pty_native.gd

@@ -0,0 +1,28 @@
+tool
+extends Node
+
+signal data_received(data)
+signal exited(exit_code, signum)
+
+
+func open(cols: int, rows: int):
+	return _not_implemented()
+
+
+func resize(cols: int, rows: int):
+	return _not_implemented()
+
+
+func get_master():
+	return _not_implemented()
+
+
+func _not_implemented() -> int:
+	var method := ""
+
+	var stack = get_stack()
+	if stack.size() >= 2 and "function" in stack[1]:
+		method = "%s()" % stack[1].function
+
+	push_error("Method %s not implemented on the current platform (%s)." % [method, OS.get_name()])
+	return ERR_METHOD_NOT_FOUND