Zed Package Manager

Package Manager that's mostly done.  Main thing it needs is documentation.  Examples of extensions can be found here: https://github.com/TheKiteEatingTree/zed-extensions

To install those extensions you have to use the raw.github urls:
- https://raw.github.com/TheKiteEatingTree/zed-extensions/master/HTMLHint/
- https://raw.github.com/TheKiteEatingTree/zed-extensions/master/Objective-C/

A few things I'm unsure about:
- The functions are all callable from the sandbox.
- UI prompts sometimes dismiss instantly.  Not sure if that's something I did wrong or something in the ui prompt code.
- /config/default/zpm/installed.json doesn't seem to be read-only in the configuration project.
- I added some code in sandbox.js to console log messages from the webview.
- update error handling - An extension could be broken if there's an error during an update.  But it should fix itself by updating again.  I didn't consider this too big of a deal, but I should fix it at some point.
- uninstall error handling - An extension could get removed from user.json but not be uninstalled.  Could still uninstall it later.  Or it could fail to delete some files that could then only be removed manually.