[staging] Support Termios operations

[milseman]

Adds a draft implementation of termios support, using the TerminalDescriptor design style akin to SocketDescriptor.

Also adds a couple fun sample commands:

Terminal size

% swift run system-samples terminal-size 
196x56

% ls
cmake           CODE_OF_CONDUCT.md  LICENSE.txt     Package.swift       README.md       Tests
CMakeLists.txt      CONTRIBUTING.md     Package.resolved    Proposals       Sources

% swift run system-samples terminal-size --border --set-rows 5 --set-columns 80
Attempting to set terminal size to 80x5...
✓ Successfully set terminal size

80x5

┌──────────────────────────────────────────────────────────────────────────────┐
│                                     80x5                                     │
│                                                                              │
│                                                                              │
└──────────────────────────────────────────────────────────────────────────────┘

% ls
cmake           LICENSE.txt     README.md
CMakeLists.txt      Package.resolved    Sources
CODE_OF_CONDUCT.md  Package.swift       Tests
CONTRIBUTING.md     Proposals

Password reader (disable echo)

% swift run system-samples password-reader
Password: 
Password read successfully (3 characters)
Password was: ***

Raw mode reader

% swift run system-samples raw-mode
Raw Mode Key Reader
==================

In raw mode, characters are available immediately without
waiting for Enter. Press 'q' to quit.

Try pressing keys, arrow keys, or special keys...

Read 3 byte(s): 0x1B 0x5B 0x43  [RIGHT ARROW]

Terminal restored to normal mode.