Speed up the update_stale function.

[hworld]

More performance fun!

I noticed that the extension output was pausing during "Checking for stale beam files." After digging in, I realized that the code was using a private API function of Mix (Mix.Utils.extract_stale) which was always returning every single file in the project had changed on every save.

I tried to track down what Mix was doing, and basically pulled in a modified flow from that project, only supporting what we need. I still don't know why it was failing to do the proper checks, but the code in this PR is only fetching the files that have changed since the last analysis.

I decided to wrap it in a timing call so we can keep an eye on it, as well as logging how many files it found that "changed." I also added a test that works correctly to make sure that this functions properly.

As for performance, the before on a sample project of mine:

Found 1325 changed files in 3799 milliseconds
Found 1325 changed files in 4221 milliseconds
Found 1325 changed files in 3970 milliseconds

And after:

Found 1 changed files in 22 milliseconds
Found 1 changed files in 32 milliseconds
Found 1 changed files in 30 milliseconds

This is code that is at the start of the flow before it can run the analysis. With this fix, the analysis after saving in my project I get error reporting within 2s, which feels really good!

[axelson]

Looks great! Thanks for removing one of the private api usages 🎉