Port multi-byte character ratio detection in UTF-8 prober confidence function from jschardet

[yinyue200]

fix #108

[304NotModified]

@rstm-sf do we think we should merge this one?

[rstm-sf]

Hello, sorry for the delay, it took a while to understand this change.

This method can be simplified to the following state:

public override float GetConfidence(StringBuilder status = null)
{
    const float like = 0.99f;

    if (numOfMBChar >= 6)
        return like;

    var mbCharRatio = (float)mbCharLen / (fullLen - basicAsciiLen);
    if (mbCharRatio > 0.6f)
        return like;

    var negative = (float)Math.Pow(ONE_CHAR_PROB, numOfMBChar * numOfMBChar);
    return like * (1f - negative);
}

I found a partial explanation in this PR aadsm/jschardet#59

But this particular change is not entirely clear to me. Asked about it here aadsm/jschardet#57 (comment)

Also, I can't understand out why it works?

[yinyue200]

For reference：aadsm/jschardet@f45b273#r47324263

[rstm-sf]

If ONE_CHAR_PROB = 0.45f, then tests will start to pass too (instead of double pow)

(... I can't understand out why it works)

[rstm-sf]

It seems to me that this is always true. Why could it be otherwise?

[304NotModified]

@yinyue200 do you think you could check the review comments? Or should be close this PR for now?