Fix default exports for Babel plugin

babel.js

···
-module.exports = require('./dist/reghex-babel.js').default;
+module.exports = require('./dist/reghex-babel.js');

package.json

···
       "require": "./dist/reghex-core.js"
     },
     "./babel": {
-      "import": "./dist/reghex-babel.mjs",
       "require": "./dist/reghex-babel.js"
     },
     "./macro": {
-      "import": "./dist/reghex-macro.mjs",
       "require": "./dist/reghex-macro.js"
     },
     "./package.json": "./package.json"

rollup.config.js

···
   },
   {
     ...base,
+    output: {
+      ...output('cjs', '.js'),
+      exports: 'default',
+    },
     input: {
       babel: './src/babel/plugin.js',
       macro: './src/babel/macro.js',