feat: Added xonsh shell support - try 1

[anki-code]

Resolve #4622

[djc]

This all seems reasonable to me, thanks! @rami3l do you want to take a look?

(Please squash all your commits.)

[anki-code]

Test instructions for the future:

docker run --rm -it rust

apt update && apt install -y python3 python3-pip
pip install --break-system-packages xonsh
xonsh

git clone -b xonsh_shell https://github.com/anki-code/rustup
cd rustup/

# https://rust-lang.github.io/rustup/dev-guide/
cargo build
mkdir home
$RUSTUP_HOME='home' $CARGO_HOME='home' target/debug/rustup-init --no-modify-path -y
source /rustup/home/env.xsh

$PATH.pop(0)  # remove '/usr/local/cargo/bin' from container
which cargo
# /rustup/home/bin/cargo
cargo --version
# cargo 1.91.1

@djc, thanks for the review! Sorry I can't do squash because the branch has commits from github editor and from local (error "commit has two parents") but check your merge button. Ordinarily it has squash option.

[djc]

Sorry I can't do squash because the branch has commits from github editor and from local (error "commit has two parents") but check your merge button. Ordinarily it has squash option.

Not here, because we have the merge queue enabled. You should be able to squash all the commits using git rebase -i.

[anki-code]

@djc I've made new PR because I faced with issues around squash - #4624

But in the repository settings there is an option - "Allow squash merging":

So you can click: