Add ctrl+enter key binding for Linux

[jborean93]

PR Summary

Adds the Ctrl+Enter key binding for Linux that defaults to AcceptLine. This allows PSReadLine to work by default on Linux terminals like Alacritty (in tmux) where a newline in a paste comes across as the Ctrl+Enter key. Not all terminals on Linux will send a newline in a paste as this key but some do. Having this key handler means that pasting a string with newlines is preserved properly rather than as

You can replicate how a newline was handled in a tty with a .NET application by running the following Python code on Linux

import os
import pty
import subprocess
import sys

parent, child = pty.openpty()
p = subprocess.Popen(
    ['pwsh', '-Command', '[Console]::ReadKey($true) | ConvertTo-Json -EnumsAsString'],
    preexec_fn=os.setsid,
    stdin=child,
    stdout=sys.stdout,
    stderr=sys.stderr,
)
stdin = os.fdopen(parent, 'wb', 0)
os.close(child)

stdin.write(b"\n")
p.communicate()

The output on PowerShell 7.4/.NET 8 is

{
  "KeyChar": "\n",
  "Key": "Enter",
  "Modifiers": "Control"
}

.NET introduced a rewrite of Console.ReadKey() in .NET 7 dotnet/runtime#72193 that changed how a \n on a tty was handled. In the past this is what it returned

{
  "KeyChar": "\r",
  "Key": "Enter",
  "Modifiers": 0
}

As there were no modifiers the \r was treated as a newline and so just worked.

I'm unsure how you wish for me to test this, any guidance there would be appreciated.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure you've added one or more new tests
• Make sure you've tested these changes in terminals that PowerShell is commonly used in (i.e. conhost.exe, Windows Terminal, Visual Studio Code Integrated Terminal, etc.)
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed at PowerShell-Docs
    • Doc Issue filed:

Microsoft Reviewers: Open in CodeFlow