public char getExpressionStart() {
        return currentState.expressionStart;
    }
Get the expression start character of the <em>current</em> state.

<p>This value should not be cached and reused across pushed and popped
states. This value is "$" by default, but it can be changed using
{@link #setExpressionStart(char)}.

@return Returns the expression start char of the current state.
smithy-utils/src/main/java/software/amazon/smithy/utils/AbstractCodeWriter.java