Feat/opml storage structure

[AngeloR]

By switching to opml we are following the standard for outliners. By
using a single file for storage, we are also making it significantly
easier to sync the outlines themselves elsewhere and also support
additional tooling around opml files.

References:

• https://opml.org/spec2.opml

[github-actions]

Build for macos-latest is done.
Uploaded Assets:
["/Users/runner/work/outliner/outliner/src-tauri/target/aarch64-apple-darwin/release/bundle/dmg/Outliner_0.0.6_aarch64.dmg","/Users/runner/work/outliner/outliner/src-tauri/target/aarch64-apple-darwin/release/bundle/macos/Outliner.app"]

[github-actions]

Build for ubuntu-22.04 is done.
Uploaded Assets:
["/home/runner/work/outliner/outliner/src-tauri/target/release/bundle/deb/Outliner_0.0.6_amd64.deb","/home/runner/work/outliner/outliner/src-tauri/target/release/bundle/rpm/Outliner-0.0.6-1.x86_64.rpm","/home/runner/work/outliner/outliner/src-tauri/target/release/bundle/appimage/Outliner_0.0.6_amd64.AppImage"]

[github-actions]

Build for windows-latest is done.
Uploaded Assets:
["D:\a\outliner\outliner\src-tauri\target\release\bundle\msi\Outliner_0.0.6_x64_en-US.msi","D:\a\outliner\outliner\src-tauri\target\release\bundle\nsis\Outliner_0.0.6_x64-setup.exe"]