feat: add isDirty method

[aadamcik]

❓ Type of change

• 👌 Enhancement (improving an existing functionality like performance)

📚 Description

This PR adds isDirty(fields) method for easier checking whether field has changed. Which could be replacement for .$isDirty

// isDirty(fields?: keyof ModelAttributes<this> | (keyof ModelAttributes<this>)[]): boolean

const user = new User()
user.username = 'test'

user.isDirty('username') // true
user.isDirty(['username']) // true
user.isDirty() // true

Which is easier to type and type-safe vs 'username' in user.$dirty and more convenient when checking multiple fields, if any is dirty.

[thetutlage]

Thanks 👍