fix(plugins): keep each stored value in its own file (#1971)

Each stored key gets its own file, so a set no longer rewrites the whole 64 MiB storage and a get no longer parses it; an older storage.json moves into per-key files on first use.

Cargo.lock

@@ -4319,6 +4319,7 @@ name = "ide-host"
 version = "0.1.38"
 dependencies = [
  "base64 0.22.1",
+ "blake3",
  "criterion",
  "dirs 6.0.0",
  "dunce",