Refactor variables, add text right align setting, add input reader

[sunjc826]

• Made some minor changes based on shellcheck's suggestions.
• Added pushd into script directory so shellcheck knows where to source (no need for # shellcheck source directive). This together with the .shellcheckrc makes hovering over variables/functions defined in simple_curses.sh in vscode work.
• Added append <text> right
• Added reading of input into the event loop, enabled by setting --enable-input. It does a bunch of somewhat "nonblocking" reads, technically read -n 1 char is blocking but it should always succeed.

Let me know what you think about the last feature! I think it is still very rough around the edges.

[sunjc826]

Addresses #32 #33

[GrownLinux]

Thank you for the changes and updates! Firstly, incorporating Shellcheck suggestions is excellent as it helps maintain clean and error-free code. Additionally, including pushden in the script directory, along with .shellcheckrc, enhances Visual Studio Code integration, making it easier for Shellcheck to locate variable and function definitions. Moreover, the newly added append right function sounds interesting.

[metal3d]

Thank a lot for this PR!