feat: export all classes

update #2

src/theta.ts

@@ -1,4 +1,12 @@
 // Import here Polyfills if needed. Recommended core-js (npm i -D core-js)
 // import "core-js/fn/array.find"
 // ...
-export default class DummyClass {}
+
+import Expression from './equations/expression'
+import Constant from './symbols/constant'
+import Variable from './symbols/variable'
+import Term from './symbols/term'
+import Operator from './symbols/operator'
+import GreekLetter from './symbols/greekLetter'
+
+export { Expression, Constant, GreekLetter, Variable, Term, Operator }