feat: Add isDeleted() method to containers

[zxch3n]

Add Container Deletion Status Check

This PR adds the ability to check if a container has been deleted through the new isDeleted() method available on all container types.

Changes

• Added isDeleted() method to the ContainerTrait trait
• Implemented isDeleted() for all container types:
  • LoroText
  • LoroMap
  • LoroList
  • LoroTree
  • LoroMovableList
  • LoroCounter
  • LoroUnknown
• Fixed tracking of deleted containers in tree operations
• Added comprehensive tests covering various deletion scenarios:
  • Tree node deletion
  • Movable list container replacement
  • Map value overwrite
  • Remote deletion sync

Testing

Added test cases to verify deletion status:

• Basic container deletion through tree operations
• Container replacement in movable lists
• Map value overwriting
• Cross-document deletion sync

Usage Example

const doc = new LoroDoc();
const map = doc.getMap("map");
const sub = map.setContainer("sub", new LoroMap());
expect(sub.isDeleted()).toBe(false);

map.set("sub", "value"); // Replace container with primitive
expect(sub.isDeleted()).toBe(true);