A11y improvements, and handle tab discarding

• Update to hterm 1.81. A11y improvements, and keyboard/mouse tweaks.
• Update to libdot 1.23. Build improvements, i18n helpers, and npm support.
• nassh: Warn before closing open SSH connection.
• FAQ: Add a section about terminal encoding.
• mkcrosh: Rewrite to simplify the build.
• Disable Automatic tab discarding.
• build: Convert concat to a python script.
• nassh: Drop nonexistent file.
• gnubby: Clear lastError from gnubby probe.
• FAQ: Add more details to the legacy cipher/mac questions.
• FAQ: Expand IPv6 zone id docs.
• fsp: Split mount logic into sep knob.
• nassh: Unify profile based connection setup.
• nassh: Make connectToArgString fully async.
• nassh: Use -1 exit status for internal JS errors.
• nassh: Make registerProtocolHandler a bit more flexible.
• FAQ: Improve omnibox docs with different launch modes.
• build: Use readlink instead of realpath in promote script.
• a11y: Make it possible to enable/disable accessibility at runtime.