Backspace deletes last readonly character

Steps to reproduct:
Start new "ConsoleControlSample.WPF" process.
Click "Run command" starts new command shell
Hit backspace on input area
Last character on output, probable a ">" character is removed.

Fix proposal:
on "richTextBoxConsole_PreviewKeyDown" method, change
```
var caretPosition = richTextBoxConsole.GetCaretPosition();
var delta = caretPosition - inputStartPos;
var inReadOnlyZone = delta < 0;
```
with this
```
var caretPosition = richTextBoxConsole.GetCaretPosition();
var delta = caretPosition - inputStartPos;
var inReadOnlyZone = delta <= 0;
```

Start the ConsoleControlSample.WPF process, run a command, and reproduce the backspace behavior in the input area. Inspect richTextBoxConsole_PreviewKeyDown and compare the current read-only-zone boundary with the proposed change. Done means backspace no longer removes the last read-only output character while input editing still works.

Written by the indexing model from the issue text.