feat(dia.Cell): new toJSON options

Description

This PR includes changes to the dia.Cell.toJSON(), now accepting two new options.

opt.ignoreDefaults (default ['attrs']) - accepts an array of attribute names to be compared to the defaults, additionally setting this option to false will result in returning all of the attributes without comparing them, while setting it to true results in a comparison of all the attributes with defaults
opt.ignoreEmptyAttributes (default false) - determines if the result should include empty attributes

These two options are also applicable to dia.Graph.toJSON() method.

This PR also includes a new util.objectDifference function used to determine the difference between two provided objects.

Documentation

https://docs.jointjs.com/api/dia/Cell/#tojson

`toJSON()`

cell.toJSON([opt])

Return a copy of the cell's attributes for JSON serialization. This can be used for persistance or serialization. Note that this method doesn't return a JSON string but rather an object that can be then serialized to JSON with JSON.stringify().

By default, opt.ignoreDefaults is set to ['attrs']. This setting ensures that only non-default attributes in the attrs object are returned, as it compares the attrs object to its defaults. To include all attributes of dia.Cell without filtering out defaults, set opt.ignoreDefaults to false. Alternatively, setting opt.ignoreDefaults to true will compare all attributes to their defaults, returning only those that differ. You can also provide an array of strings for opt.ignoreDefaults, specifying a list of attributes to compare against their defaults.

Additionally, opt.ignoreEmptyAttributes defaults to false, which may include empty objects in the JSON output. To exclude empty objects, set opt.ignoreEmptyAttributes to true.

const rectangle = new shapes.standard.Rectangle();

rectangle.toJSON();
// {
//     "type": "standard.Rectangle",
//     "attrs": {},
//     "position": {
//         "x": 0,
//         "y": 0
//     },
//     "size": {
//         "width": 1,
//         "height": 1
//     },
//     "angle": 0,
//     "id": "..."
// }

rectangle.toJSON({ ignoreDefaults: true });
// {
//     "type": "standard.Rectangle",
//     "attrs": {},
//     "position": {},
//     "size": {},
//     "id": "..."
// }

rectangle.toJSON({ ignoreDefaults: true, ignoreEmptyAttributes: true });
// {
//     "type": "standard.Rectangle",
//     "id": "..."
// }

rectangle.toJSON({ ignoreDefaults: ['attrs', 'size'] });
// {
//     "type": "standard.Rectangle",
//     "attrs": {},
//     "position": {
//         "x": 0,
//         "y": 0
//     },
//     "size": {},
//     "angle": 0,
//     "id": "..."
// }

https://docs.jointjs.com/api/dia/Graph/#tojson

toJSON()

graph.toJSON([opt])

This method accepts an optional opt object, the opt.cellAttributes accepts same options as the dia.Cell.toJSON() method, which will impact the result of the toJSON() method.

https://docs.jointjs.com/api/util/

objectDifference()

util.objectDifference(object, base, [opt])

Return an object that represents the difference between object and base.

object	Object	The object to compare
base	Object	The object to compare with
[opt.maxDepth]	number	The maximum depth to compare

clientIO / joint