Went for option 2 proposed here:
https://github.com/microsoft/terminal/issues/10140#issuecomment-845193132.
Disabling back space in the nested entry didn't felt as the nicest
solution. Instead now all state that keeps track of nested commands is
cleared when switching beteen modes.
## Validation Steps Performed
- Validated the specified issue is fixed by this change:. now after
entering a sub command and hitting backspace the palette no longer
shows the sub command item (here `< Select color scheme...` ).
- Validated that switching between all modes (command line, actions, tab
search & tab switch) still work as expected.
- Validated as well that all modes still work as expected.
Didn't add unit tests, but happy to try that out if this would be
required.
Closes#10140
dd348dccda