Improve Apple Silicon compatibility

[mafredri]

On M1 Macs, the Homebrew path has changed to /opt/homebrew so we check at which path zsh is present.

Additionally we try to install into standard unix path if present, however, this will likely only help people installing via sudo.

Finally, the error text has been updated to prioritize updating .zshrc instead of doing a manual install.

Fixes #584.

---

Example output of updated error text (where /usr/local/bin/zsh and /opt/homebrew/bin/zsh is not present):

❯ npm run --silent postinstall
ln: /usr/share/zsh/site-functions/prompt_pure_setup: Operation not permitted

ERROR: Could not automagically symlink the prompt. You can either:

1. Add the following to your `.zshrc`:

    fpath+=('/Users/maf/Code/pure/functions')

2. Or check out the readme on how to do it manually: https://github.com/sindresorhus/pure#manually