Retrieves a value from a form element located by CSS or XPath and returns it to test.
Resumes test execution, so **should be used inside async function with `await`** operator.
If more than one element is found - value of first element is returned.

```js
let email = await I.grabValueFrom('input[name=email]');
```
@param {CodeceptJS.LocatorOrString} locator field located by label|name|CSS|XPath|strict locator.
@returns {Promise<string>} attribute value