fix: add a timeout to the res.sep when discovering new dependency

[elevatebart]

To avoid returning an erroring file to import in the browser, I choose to wait 5 seconds before returning.
It gives the browser the time it needs to refresh, instead of responding so that request with an invalid empty MIME response.
If the page fails to reload, after 5s the empty invalid response is still sent.

closes #2525

[lmiller1990]

Anything else you see that needs attention prior to merging this @yyx990803 or @antfu?

[Shinigami92]

question: I may be wrong here, but currently the code reads like it always waits statically 5000ms and then respond with a 408.
What makes that it could respond earlier?
Maybe I just misinterpret the issue and just reading the code here

[elevatebart]

@Shinigami92 You are absolutely right to ask.
This branch of code is reached when a dependency needs a complete refresh of the page.
Even if the query never responds, the page is going to be refreshed and the query forgotten.

Chronologically:

• Browser asks for a dependency that is not yet optimized
• Server looks for it and starts optimization
• We reach line 57 of this file
• The server finishes optimizing the new dependency
• Server sends a WebSocket message to the browser to refresh
• Optimization process resolves the promise on line 57/58
• Here is the change in this PR:
  • Before this PR, an empty response was given to the browser before it could refresh. This triggered the error mentioned in the ticket
  • After this PR: If the browser has not refreshed the page after 5 seconds we just drop the request

I hope this chronology clarifies the situation.