fix: onRatingUpdate called with wrong state

[asci-00]

Thank you for providing flutter_rating_bar.
There is something I would like to contribute while using this, so I would like to request a PR.

Problem

• When the iconRating is changed, onRatingUpdate is not called immediately, so if the _rating state changes quickly, the iconRating value referenced by _onDragUpdate / _onDragEnd may not be up to date.

Describe your changes

• Separated ratingOffset into commonly used states rather than individual methods. (_ratingOffset)
• IconRating did not seem to need to be dependent on the index, so I modified it to be a getter based on the _rating value independently. It seems to be working well in my code. Could you please give me some feedback on this?
• Modified the no-rating / half-rating / full-rating conditions by modifying the iconRating calculation method. This part may also be different from what you intended, so please give me feedback and I will correct it.

Issue ticket number and link

#76

[asci-00]

@sarbagyastha Could you review this PR?