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The flag '-h' isn't universal across implementation. According to FreeBSD man page for ln you can use 'n'.

Force the mark to point to the new dir, replacing the old one.

Fixes #7195

Changed aliases from
```
alias vsce='code --extensions-dir'
alias vsced='code --disable-extensions'
```
to
```
alias vsced='code --extensions-dir'
alias vscde='code --disable-extensions'
```

`vsce` is the command line to publish extensions:
https://code.visualstudio.com/docs/extensions/publish-extension

This commit triggers precmd and chpwd hook functions iff we changed directory.

This has the same behavior as zsh's hook function execution, which tries to run
the functions in the order specified and silently ignores any function that
does not exist.

See http://zsh.sourceforge.net/Doc/Release/Functions.html#Hook-Functions

Also moved duplicate nopushdminus logic to the `switch-to-dir` function.

Detect system Ruby in default `PATH`, and provide "system" completion if found.

Tested with [the Dockerfile](https://gist.github.com/franklinyu/b8deda6a5093a17b575679b5808b371f).