devcontainer configuration update

[Yakiyo]

This commit changes the current setup but does not add or remove anything major.

• rename setup.sh to script.sh: If the file is named setup.sh github uses that instead of using any user defined dotfile repo. In my case i have a dotfiles repo that adds my wakatime api key to wakatime config file. But due to the .devcontainer/setup.sh file, it gets ignored which is kinda bothersome.
• Use the direct path to rustup and cargo in script.sh: For some reason installing rustup does not immediately load the ~/.cargo/bin directory to path. Trying to source the .bashrc/.zshrc file exits the setup file for some reason. ~~As such using the direct path to cargo and rustup works.~~ Re-exporting the PATH var after adding cargo bin dir works.
• Use cargo-binstall: For installing cargo-expand and cargo-edit, its faster to use cargo-binstall. cargo-binstall itself is easy to install using a one-liner shell script and installation of the cargo-plugins is also way faster as theres no need to compile the plugins
• Change .devcontainer.json: Nothing major in it. It just moves the settings and extensions property inside customizations.vscode cause the devcontainer extension just screams the warnings otherwise.

I have tried and tested the above configuration and have ensured it works. I've also considered replacing oh-my-zsh with oh-my-posh since features are nearly identical for both but oh-my-posh would work with other shells too in case necessary. But not sure wether that would be supported by the repo authors.