feat: specify shell via $AICHAT_SHELL

[sigoden]

On Unix-like OS, aichat detects the shell via $SHELL, then falls back to sh.
On Windows, aichat detects shell via PSModulePath, then falls back to cmd.

After this PR is merged, aichat will first detect the shell via AICHAT_SHELL.
Even if you use fish in Cygwin on Windows, you can ensure it works properly by setting AICHAT_SHELL.

close #864

[sigoden]

Tested the following:

# Git Bash
export AICHAT_SHELL="C:\\Program Files\\Git\bin\\bash.exe"

# Msys2 Bash
export AICHAT_SHELL="C:\\msys64\\usr\\bin\\bash.exe"

# Msys2 Zsh
export AICHAT_SHELL="C:\\msys64\\usr\\bin\\zsh.exe"

# Msys2 Fish
set -x AICHAT_SHELL "C:\\msys64\\usr\\bin\\fish.exe"