[Feat]: More sophisticated way of managing states

### Describe the feature / 功能描述

I'd like to add / remove a single state from a node.

Currently this could be done the following way:
```ts
graph.setElementState(node.id, [...node.states, add_state]);
graph.setElementState(node.id, node.states.filter(s => s !== rm_state));
```

The graph api could be improved by adding `graph.addState(node, state)` and `graph.removeState(node, state)`.

### Are you willing to contribute? / 是否愿意参与贡献？

✅ Yes / 是

Start by locating the Graph API implementation and tests for setElementState, then review how node states are represented and updated. Done means the API supports adding or removing one state from a node and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.