Remove the insane dependency bloat

[niansa]

So, downloading a binary of Zed and scrolling through its internal symbols reveals that the release builds contain a LOT of stuff that should REALLY be user-installed, not shipped:

• WebRTC
• Audio processing (echo cancellation etc.)
• Video codecs (VP9, VP8, ...)
• Image processing
• Some AWS client?
• An entire JIT
• ... tons of other stuff

Which bloats the total binary size up to over 250 MiB! Which puts Zed right in class with an Electron application despite it not being a web browser.
I feel like this has probably happened because dependencies were added without further consideration of consequences.

If something like a JIT is really needed, loading the implementation from system-wide libraries (if installed) should be considered instead.