fix:check input values to avoid zero million or impossible values

[KyrieCheng-China]

problem: there is no check with input value. for example, user can input 00,012,00000012, like these impossible values.

solution: add check when there is a change of text, if it's first two characters are match 0[0-9],the input will be rejected, means it will keep "0" instead of "00".

Fixes #41

[forzzzzz]

Hi, there is a class that is responsible for entering characters app/src/main/java/com/forz/calculator/utils/InsertInExpression.kt

    fun enterDigit(digit: String, inputEditText: ExpressionEditText) {
        val string = inputEditText.text.toString()

        deleteSelection(inputEditText, inputEditText.selectionStart, inputEditText.selectionEnd)

        val cursorPositionStart = inputEditText.selectionStart

        val stringAfterCursor: String = stringAfterPosition(string, cursorPositionStart)
        val operatorAfterDigit: String = if (afterDigitArray.any { stringAfterCursor.startsWith(it) }) DefaultOperator.Multiply.text else ""

        val stringBeforeCursor: String = stringBeforePosition(string, cursorPositionStart)
        val operatorBeforeDigit: String = if (beforeDigitArray.any { stringBeforeCursor.endsWith(it) }) DefaultOperator.Multiply.text else ""

        val textInsert = "$operatorBeforeDigit$digit$operatorAfterDigit"
        inputEditText.text?.insert(cursorPositionStart, textInsert)

        correctPositionIfOperatorAfterNumberIsSpace(operatorAfterDigit, inputEditText)
    }

function for entering digits