GHSA-78qv-3mpx-9cqq: NiceGUI vulnerable to XSS via Code Injection during client-side element function execution

Use `json.dumps()` for proper escaping of method and property names in `run_method()` and `get_computed_prop()`, and remove the `eval()` fallback from the `runMethod()` function in `nicegui.js` so that invalid method names raise an error instead of being executed as code. Code that previously relied on passing JavaScript functions as method names should use `ui.run_javascript()` instead, for example: `row = await ui.run_javascript(f'return getElement({grid.id}).api.getDisplayedRowAtIndex(0).data')` instead of `row = await grid.run_grid_method('g => g.getDisplayedRowAtIndex(0).data')`.