feat: add tcsh install support

[obust]

This addresses #897

Before

case supported shell: bash

$ echo $SHELL
/bin/bash

$ bash ./install/install.sh
This script will automatically download and install Pixi (latest) for you.
Getting it from this url: https://github.com/prefix-dev/pixi/releases/latest/download/pixi-aarch64-apple-darwin.tar.gz
The binary will be installed into '/Users/romain/.pixi/bin'
######################################################################## 100.0%
Updating '/Users/romain/.bash_profile'
Please restart or source your shell.

case unsupported shell: tcsh

$ echo $SHELL
/bin/tcsh

$ bash ./install/install.sh
This script will automatically download and install Pixi (latest) for you.
Getting it from this url: https://github.com/prefix-dev/pixi/releases/latest/download/pixi-aarch64-apple-darwin.tar.gz
The binary will be installed into '/Users/romain/.pixi/bin'
######################################################################## 100.0%
Unsupported shell: bash
Please restart or source your shell.

Now

case supported shell: tcsh

$ echo $SHELL
/bin/tcsh

$ bash ./install/install.sh
This script will automatically download and install Pixi (latest) for you.
Getting it from this url: https://github.com/prefix-dev/pixi/releases/latest/download/pixi-aarch64-apple-darwin.tar.gz
######################################################################### 100.0%
The 'pixi' binary is installed into '/Users/romain/.pixi/bin'
Updating '/Users/romain/.tcshrc'
Please restart or source your shell.

See references:

• append path in .tcshrc: https://www.peachpit.com/articles/article.aspx?p=31442&seqNum=4

case unsupported shell: ksh

$ echo $SHELL
/bin/ksh

$ bash ./install/install.sh
This script will automatically download and install Pixi (latest) for you.
Getting it from this url: https://github.com/prefix-dev/pixi/releases/latest/download/pixi-aarch64-apple-darwin.tar.gz
######################################################################### 100.0%
The 'pixi' binary is installed into '/Users/romain/.pixi/bin'
Could not update shell: ksh
Please permanently add '/Users/romain/.pixi/bin' to your $PATH to enable the 'pixi' command.

I believe the error message is clearer and the user can proceed on their own to enable pixi. See issue #897 for more explanations own why I find the previous error confusing/misguiding.

[wolfv]

Thanks! This looks good to me 👍

[ruben-arts]

Thanks @obust