Add canonicalEventSerialization preference for stable, diff-friendly event JSON

Problem

GDevelop's event serialization omits properties that hold default values — disabled, folded, inverted, await are only written to JSON when true; events, variables, subInstructions, loopIndexVariable, etc. are only written when non-empty. This causes two kinds of noise in git diffs:

1. Line insertions / deletions — toggling a boolean flag adds or removes a line, shifting every subsequent line in the block.
2. Unstable key order — JSON object keys are emitted in code-execution order, which differs between event types and changes when events are reorganised.

Both issues make per-event diffs hard to review in version-controlled projects.

Solution

Add a project-level preference canonicalEventSerialization (opt-in, default false). When enabled:

• All conditionally-omitted properties are always written with their default values (false, "", []), so toggling a flag produces a single-line value change, not an insertion/deletion.
• All JSON object keys are written in alphabetical order, giving a stable, predictable layout regardless of event type or serialization order.

The preference is controlled via gdevelop-settings.yaml (already read by the IDE on project open):

# gdevelop-settings.yaml
preferences:
  canonicalEventSerialization: true