Add Flatpak build system and support (#12006)

ping #6687 

This is the third iteration of this PR ([v2
here](https://github.com/zed-industries/zed/pull/11949)) and uses a
different approach to the first two (the process wrapper lib was a
maintainability nightmare). While the first two attempted to spawn the
necessary processes using flatpak-spawn and host-spawn from the app
inside the sandbox, this version first spawns the cli binary which then
restart's itself *outside* of the sandbox using flatpak-spawn. The
restarted cli process than can call the bundled app binary normally,
with no need for flatpak-spawn because it is already outside of the
sandbox. This is done instead of keeping the cli in the sandbox because
ipc becomes very difficult and broken when trying to do it across the
sandbox.

Gnome software (example using nightly channel and release notes
generated using the script):
<img
src="https://github.com/zed-industries/zed/assets/81528246/6391d217-0f44-4638-9569-88c46e5fc4ba"
width="600"/>

TODO in this PR:
- [x] Bundle libs.
- [x] Cleanup release note converter.

Future work:

- [ ] Auto-update dialog
- [ ] Flatpak auto-update (complete 'Auto-update dialog' first)
- [ ] Experimental
[bundle](https://docs.flatpak.org/en/latest/single-file-bundles.html)
releases for feedback (?).

*(?) = Maybe / Request for feedback*

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

docs/src/development/linux.md

@@ -70,6 +70,18 @@ cargo test --workspace
 
 Zed has basic support for both modes. The mode is selected at runtime. If you're on wayland and want to run in X11 mode, you can set `WAYLAND_DISPLAY='' cargo run` to do so.
 
+## Flatpak
+
+> [!WARNING]
+> Zed's current Flatpak integration simply exits the sandbox on startup. Workflows that rely on Flatpak's sandboxing may not work as expected. 
+
+To build & install the Flatpak package locally follow the steps below:
+
+1. Install Flatpak for your distribution as outlined [here](https://flathub.org/setup).
+2. Run the `script/flatpak/deps` script to install the required dependencies.
+3. Run `script/flatpak/bundle-flatpak`.
+4. Now the package has been installed and has a bundle available at `target/release/{app-id}.flatpak`.
+
 ## Troubleshooting
 
 ### Can't compile zed