DNM Add flatpak manifest: offline theia build not working

[LorbusChris]

This change adds a flatpak manifest.

It is not meant for inclusion here but rather meant to help fix remaining issues with this way of building theia-ide in an offline-manner.

Eventually, my plan is to create a dedicated repo for this on Flathub.

Related to: #360

---

I'm running into two distinct issues with the offline building of theia-ide as required for flatpaks.
I'll focus on the building of the electron app for now as that's what's going to be needed for the flatpak container.

This specific issues arises here when theia build --app-target="electron" is invoked (the yarn -s rebuild beforehand finishes successfully).

theia build is trying to call out to github and we can't have that in an offline build. If anyone could help solve this I'd highly appreciate it.

$ flatpak run org.flatpak.Builder build-init org.eclipse.theia-ide.yaml --force-clean --install-deps-from=flathub --user --install
[snip]
theia-ide-electron-app: ✔ Rebuild Complete
theia-ide-electron-app: npm http fetch GET https://registry.yarnpkg.com/npm attempt 2 failed with EAI_AGAIN
theia-ide-electron-app: npm info ok
theia-ide-electron-app: RequestError: getaddrinfo EAI_AGAIN github.com
theia-ide-electron-app:     at ClientRequest.<anonymous> (/run/build/theia-ide/node_modules/got/dist/source/core/index.js:970:111)
theia-ide-electron-app:     at Object.onceWrapper (node:events:632:26)
theia-ide-electron-app:     at ClientRequest.emit (node:events:529:35)
theia-ide-electron-app:     at origin.emit (/run/build/theia-ide/node_modules/@szmarczak/http-timer/dist/source/index.js:43:20)
theia-ide-electron-app:     at TLSSocket.socketErrorListener (node:_http_client:501:9)
theia-ide-electron-app:     at TLSSocket.emit (node:events:517:28)
theia-ide-electron-app:     at emitErrorNT (node:internal/streams/destroy:151:8)
theia-ide-electron-app:     at emitErrorCloseNT (node:internal/streams/destroy:116:3)
theia-ide-electron-app:     at process.processTicksAndRejections (node:internal/process/task_queues:82:21)
theia-ide-electron-app:     at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:107:26) {
theia-ide-electron-app:   code: 'EAI_AGAIN',
theia-ide-electron-app:   timings: {
theia-ide-electron-app:     start: 1725423186506,
theia-ide-electron-app:     socket: 1725423186509,
theia-ide-electron-app:     lookup: 1725423186510,
theia-ide-electron-app:     connect: undefined,
theia-ide-electron-app:     secureConnect: undefined,
theia-ide-electron-app:     upload: undefined,
theia-ide-electron-app:     response: undefined,
theia-ide-electron-app:     end: undefined,
theia-ide-electron-app:     error: 1725423186510,
theia-ide-electron-app:     abort: undefined,
theia-ide-electron-app:     phases: {
theia-ide-electron-app:       wait: 3,
theia-ide-electron-app:       dns: 1,
theia-ide-electron-app:       tcp: undefined,
theia-ide-electron-app:       tls: undefined,
theia-ide-electron-app:       request: undefined,
theia-ide-electron-app:       firstByte: undefined,
theia-ide-electron-app:       download: undefined,
theia-ide-electron-app:       total: 4
theia-ide-electron-app:     }
theia-ide-electron-app:   }
theia-ide-electron-app: }
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

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 >  Lerna (powered by Nx)   Running target build:prod for project theia-ide-electron-app failed

   Failed tasks:
   
   - theia-ide-electron-app:build:prod
   
   Hint: run the command with --verbose for more details.

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
Error: module theia-ide: Child process exited with code 1