refactor: detect current shell

[baszalmstra]

Refactored the shell detection code. The following changes were made.

• Added a function to detect the current shell based on the SHELL environment variable.
• Added a function to detect the current shell based on the parent process name (behind a feature flag).
• Removed the ShellEnum::detect_from_environment() method and moved implementation into ShellEnum::default().

To determine an appropriate current shell use:

let shell = ShellEnum::from_env() // First try to detect from $SHELL
    .or_else(ShellEnum::from_parent_process())  // Otherwise check the parent process
    .unwrap_or_default();  // Otherwise fall back to the os's default

Fix #216