Shell type for the terminal: NULL or "default" to use the
shell selected in Global Options. For Microsoft Windows, alternatives are
"win-cmd" for 64-bit Command Prompt, "win-ps" for 64-bit PowerShell,
"win-git-bash" for Git Bash, or "win-wsl-bash" for Bash on Windows Subsystem
for Linux. On Linux, Mac, and RStudio Server "custom" will use the custom
terminal defined in Global Options. If the requested shell type is not
available, the default shell will be used, instead.