Export ExprString (#189)

Author: zachicecreamcohn

src/index.ts

@@ -6,7 +6,7 @@
 
 export {ExprError} from './errors';
 export {Expression} from './expression';
-export {ExprElement, ExprIdentifier, ExprNumber, ExprOperator} from './elements';
+export {ExprElement, ExprIdentifier, ExprNumber, ExprOperator, ExprString} from './elements';
 export {ExprFunction} from './functions';
 export {CONSTANTS as HILBERT_CONSTANTS, SPECIAL_IDENTIFIERS, isSpecialFunction} from './symbols';
 export {hasZero, Interval, isWhole, width} from './eval';