Issue title: Remove Babel auto-switch
 Issue body: AFAICT Flems automatically switches js files to Babel transpilation if it encounters a parse error (assuming new language feature?). The feature trigger is misguided – casually typing out Javascript with constant evaluation is bound to generate plenty of parse errors – but also the Babel config shipped is less featured than current Chrome, so I typically end up with `regeneratorRuntime is not defined` if I'm using `await` or generators, even though the browser I'm using supports these natively.
 Comments: 
 Comment 0: Yeah, this has annoyed and confused me quite some times now as well. Let's get rid of that automagically bablification!
{'source': 'GithubIssues', 'repo_name_with_owner': 'porsager/flems', 'issue_id': 924748563, 'repo_stars': 377.0}