Move terminal.gd and pty.gd to godot_xterm directory

Makes for pretty paths when extending scripts:
`extends "res://addons/godot_xterm/terminal.gd"`
vs.
`extends "res://addons/godot_xterm/nodes/terminal/terminal.gd"`

Currently "res://addons/godot_xterm/pty.gd" is acutally `pty_unix.gd`.
This is okay for now as the PTY node is only supported on Unix
platforms. However, we will need to sort it out when adding Windows
support as part of #25.

Also remove the GDXterm namespace.

project.godot

@@ -8,14 +8,9 @@
 
 config_version=4
 
-_global_script_classes=[ {
-"base": "Reference",
-"class": "GDXterm",
-"language": "GDScript",
-"path": "res://addons/godot_xterm/namespace.gd"
-} ]
+_global_script_classes=[  ]
 _global_script_class_icons={
-"GDXterm": ""
+
 }
 
 [application]