Document that some shell plugins can break VS Code Shell integration

Summary

Some shell plugins, notably popular zsh theme powerlevel10k break VS Code shell integration such that consecutive commands fail to send their exit status back to the LSP.

We should document this and potential work arounds for users.

Work arounds

If a specific plugin is the cause, conditionally check if you are in a VS Code terminal before loading it:

[[ "$TERM_PROGRAM" != "vscode" ]] && <load prompt theme>

(adapted from https://code.visualstudio.com/docs/terminal/shell-integration#_manual-installation)