The browser is currently restricted in it's ability to open external editors for files, although the experimental local folder projects might work around.
I generally advise that you basically just have function calls in event sheet scripts, then have the actual functions in script files. Makes it easier to work with.
The text editor we use does have an automatic indentation option, but it's currently turned off as it incorrectly indents curly braces if you follow the brace on newline style. We could make this configurable, but it would likely be off by default.
I don't know if you've noticed gdevelop built-in JS script editor in the timesheet. I think it's great. Although the experience of gdevelop is not as good as that of C3, I think its built-in script editor can be used for reference.