Update license information

- Renamed LICENSE -> LICENSE.md as it contains markdown, and added
  contact address.
- Adds THIRDPARTY_NOTICES.txt and THIRDPARTY_NOTICES_nopty.txt which
  bundles licenses of all third-party components for versions of
  GodotXterm compiled with and without PTY node support.
- Adds misc/gen_3rdparty_licenses.sh file to generate the above
  mentioned text files.
- Seperated node_pty and tmux licenses.
- Updated libuv submodule to use version that contains additional license
  information.
- Updated README to reflect these changes.

LICENSE.md

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2020-2021, Leroy Hopson <<godotxterm@leroy.geek.nz>> and [contributors](https://github.com/lihop/godot-xterm/contributors).
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.