Migrate to python-like APIs

[rainyl]

The implementation of gocv is not friendly to cache exception on ther language, so I opened #13 to refactor the entire APIs.

I am a heavy python user, so with the development, I decided to make the APIs of opencv_dart more concise and compitible with python to switch between the two languages more easily. And now, this work have almost finished, in the branch catch-exceptions.

Here is an exemple:

// old
void cvtColor(Mat src, Mat dst, int code);
// new
Mat cvtColor(Mat src, int code, {Mat? dst});

// then usage will change from:
cvtColor(src, dst, cv.COLOR_BGR2GRAY);
// to:
cvtColor(src, cv.COLOR_BGR2GRAY, dst: dst);
// or:
final dst = cvtColor(src, cv.COLOR_BGR2GRAY);

So, now I am considering how to publish the new APIs, it seems the old versions already have many users, publish a set of nearly totally different APIs may cause heavy negative impacts on them, but publish a new package may confuse users too, then I opened this vote to listen to your opinions.