Add new PTY node (replaces Pseudoterminal node)

Uses fork of node-pty native code for forking pseudoterminals.
Uses libuv pipe handle to communicate with the child process.

- Paves the way for cross-platform (Linux, macOS and Windows) support.
- Renames Pseudoterminal to PTY (which is much easier to type and spell :D).
- Better performance than the old Pseudoterminal node. Especially when
  streaming large amounts of data such as running the `yes` command.
- Allows setting custom file, args, initial window size, cwd, env vars
  (including important ones such as TERM and COLORTERM) and uid/gid
  on Linux and macOS.
- Returns process exit code and terminating signal.

test/integration/uv_utils/uv_utils.test.gd

@@ -0,0 +1,30 @@
+extends "res://addons/gut/test.gd"
+
+const LibuvUtils := preload("res://addons/godot_xterm/nodes/pty/libuv_utils.gd")
+
+
+class TestGetOSEnviron:
+	extends "res://addons/gut/test.gd"
+
+	const EMPTY_VAR = "GODOT_XTERM_TEST_EMPTY_ENV_VAR"
+	const TEST_VAR = "GODOT_XTERM_TEST_ENV_VAR"
+	const TEST_VAL = "TEST123"
+
+	var env: Dictionary
+
+	func before_each():
+		assert(OS.set_environment(EMPTY_VAR, ""))
+		assert(OS.set_environment(TEST_VAR, TEST_VAL))
+		env = LibuvUtils.get_os_environ()
+
+	func test_has_empty_var():
+		assert_has(env, EMPTY_VAR)
+
+	func test_empty_var_has_empty_val():
+		assert_eq(env[EMPTY_VAR], "")
+
+	func test_has_test_var():
+		assert_has(env, TEST_VAR)
+
+	func test_test_var_has_correct_val():
+		assert_eq(env[TEST_VAR], TEST_VAL)