ensure version is typed as string instead of the literal __VERSION__

src/compiler/index.ts

@@ -3,5 +3,5 @@ export { default as parse } from './parse/index';
 export { default as preprocess } from './preprocess/index';
 export { walk } from 'estree-walker';
 
-export const VERSION = '__VERSION__';
+export const VERSION: string = '__VERSION__';
 // additional exports added through generate-type-definitions.js