Fixes https://github.com/zed-industries/zed/issues/7865

* bind default prettier (re)installation decision to
`prettier_server.js` existence
* ensure the `prettier_server.js` file is created last, after all
default prettier packages installed
* ensure that default prettier directory exists before installing the
packages
* reinstall default prettier if the `prettier_server.js` file is
different from what Zed expects

Release Notes:

- Fixed incorrect default prettier installation process