[preprocessor.coverage]: 'import' and 'export' may appear only with 'sourceType: "module"'

karma.conf.js
```
...
files: [
{ pattern: 'js/add.js', type: 'module' },
{ pattern: 'test/add.spec.js', type: 'module' }
],
preprocessors: {
'./js/add.js': ['coverage']
},
...
```
add.js
```
let add = n => ++n;
export { add };
```
add.spec.js
```
import { add } from '../js/add.js';
describe('test', function() {
it('add', function() {
expect(add(2)).toBe(3);
});
});
```
文件以引用，
使用了import，报错不知到怎么处理，
更换插件就好了
```
preprocessors: {
'./js/add.js': ['karma-coverage-istanbul-instrumenter']
},
coverageIstanbulInstrumenter: {
esModules: true
},
```

Reproduce the error with the karma.conf.js configuration and the example js/add.js and test/add.spec.js files, focusing on the coverage preprocessor handling of module scripts. Compare the failing coverage setup with the shown karma-coverage-istanbul-instrumenter configuration using esModules: true; done means module imports and exports are instrumented without the sourceType error.

Written by the indexing model from the issue text.