refactor(dia.ElementView): extract snap-to-grid logic into dedicated method

[kumilingus]

Description

This change enables customization of the snap-to-grid behavior, allowing for:

• Snapping to the element's center (or to any point)
• Using key modifiers to adjust snapping behavior

import { dia, g } from '@joint/core';

const paper = new dia.Paper({
    /* ... */
    elementView: jdia.ElementView.extend({
        snapToGrid: function(_evt, x, y) {
            const gridSize = this.paper.options.gridSize;
            const { width, height } = this.model.size();
            // snap to the center of the element to the grid
            const snapX = g.snapToGrid(x + width / 2, gridSize);
            const snapY = g.snapToGrid(y + height / 2, gridSize);
            return { x: snapX - width / 2, y: snapY - height / 2 };
        }
    })
});