Added: possibility to turn off TextSelector in Select and MultiSelect; possibility to select empty value when default value is provided in InputForm.

[radekRadekk]

Hello,

I wanted to turn text selector off in multiselect input. I didn't find the option to do it, so I created this pull request.
I also found this issue: https://github.com/shibayan/Sharprompt/issues/297, so I covered it as well (hopefully).
To sum up:

1. Added possibility to turn off text selector in SelectInput and MultiSelectInput.
2. Added possibility to select empty value in InputForm when default value is provided. If this mode is on Enter select empty value (or value which is typed/inserted) and Tab insert default value. If not, the behaviour is the same as now - Enter select default value.

I recoreded short demo:

Sharprompt_demo.mp4

Please take a look and think if you want to include this PR in your library. Feel free to leave me comments about code quality, style etc.