Move editor UI settings out of the locally saved project file

[prafulaa]

Fixes #1983

Problem

The editor (IDE) settings of a project — scene zoom, grid settings, window mask, etc. — are stored inside the project file (as uiSettings on each layout and editionSettings on each external layout / custom object variant). This is user specific data, so:

• it is exported with the game, adding unnecessary content to it;
• it is rewritten whenever someone merely moves around in a scene, generating noise and conflicts for people collaborating on a project with git.

Solution

When a project is saved locally, its editor settings are now moved to a sidecar file stored next to the project file, in a .gdevelop folder:

MyGame/
├── game.json                          <- no more user specific data
└── .gdevelop/
    └── game.editor-settings.json      <- editor settings, one file per project file

• Teams can gitignore .gdevelop/ (a single entry) to stop sharing/committing these settings. Projects saved in the same folder get distinct sidecar files.
• When the project is opened, the sidecar settings are read back into the project, so the editors behave exactly as before.
• Migration is automatic and lossless: a project still containing editor settings (last saved by an older version, or an autosave — which keeps embedding them for recovery) opens with its embedded settings, which are then moved to the sidecar on the next save.
• The sidecar is written before the project file: if it cannot be written, the save fails rather than silently losing the settings. A stale sidecar is removed when there is nothing left to store.
• Cloud/online storage providers are untouched: editor settings stay in the project, as there is no "gitignore" concept there.
• Note: the settings remain in memory (and thus in previews/exports payload), as the in-game editor relies on them (sceneData.uiSettings) to restore the editor state.

Implementation

• New LocalEditorSettingsSidecar.js in LocalFileStorageProvider: computes the sidecar path (<project folder>/.gdevelop/<project file name>.editor-settings.json), extractProjectEditorSettings (finds and removes uiSettings/editionSettings from a serialized project) and applyProjectEditorSettings (fills them back, never overriding settings already present in the project).
• LocalProjectWriter writes the sidecar (and removes it when empty) during save — for both single-file and folder projects (extraction happens before splitting, so partial files stay clean too).
• LocalProjectOpener re-applies the sidecar content when the project (and autosave recovery) is opened.

Tests

• New LocalEditorSettingsSidecar.spec.js (8 tests): sidecar path mapping per project, extraction/stripping of all three settings kinds (layouts, external layouts, custom object default variant + variants), round-trip restoration, null result when nothing to store, non-destructive application (embedded settings win), robustness against malformed project/sidecar content.
• Validated against a real serialized game file (GDJS/tests/games/Text.json): extraction strips the settings, round-trip restores identical values, and the rest of the project JSON is untouched.
• Full newIDE test suite, ESLint, Prettier and Flow pass locally.