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>

.gitignore

@@ -7,6 +7,8 @@
 /script/node_modules
 /crates/theme/schemas/theme.json
 /crates/collab/seed.json
+/crates/zed/resources/flatpak/flatpak-cargo-sources.json
+/dev.zed.Zed*.json
 /assets/*licenses.md
 **/venv
 .build
@@ -25,3 +27,4 @@ DerivedData/
 .blob_store
 .vscode
 .wrangler
+.flatpak-builder