Allows Zed to open custom `zed://` links (redirects from
https://zed.dev/channels) on Linux used XDG MIME types.

This PR also allows the CLI to be able to open Zed (`zed://`) URIs
directly instead of executing the main executable in
`/usr/libexec/zed-editor`.


Release Notes:

- Linux: Allow `zed.dev/channel` (`zed://`) URIs to open on Linux
- CLI: Ability to open URIs from the command line

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>