Sandcastle gallery build path fix

[ggetz]

Description

Cleans up some rough edges that I overlooked in #12854.

• Fixes build paths so that output is put in the right place for both root cesium development as well as sandcastle development.
• Fixes the gallery file watcher and rebuild which is part of npm start.
• Does a bit of cleanup to build.js while I was in there: I noticed that it was running some async functions as a side effect. That means they were running even when importing a totally unrelated function.

Issue number and link

Fixes #12867

Testing plan

1. From a clean repository state, run npm install and npm run build. The gallery files should be built to Apps/Sandcastle2, not a parent directory as in the original bug report.
2. Run npm start. Update a gallery file in packages/sandcastle/gallery and be sure the updates are rebuilt and reflected in http://localhost:8080/Apps/Sandcastle2/index.html.
3. Change your working directory to packages/sandcastle. Ensure that running npm run dev build the gallery files to the packages/sandcastle/public directory, and that the app loads examples as expected.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @ggetz!

✅ We can confirm we have a CLA on file for you.

[jjhembd]

On my WSL Ubuntu build, this PR fixes the issue from #12867:

• No spurious Apps directory created outside the cesium folder
• Working "Sandcastle2" link on the localhost page

[jjspace]

Thanks @ggetz sorry this took a bit to get to.

I think the code changes look good. Maybe small reworks as we do other build changes but these fix the current issues.
With the confirmation from @jjhembd that this works on Windows I'm happy to merge.

[jjspace]

Actually @ggetz I went to get a picture of the slow gallery list build time I mentioned in person and realized there's an issue with the local server rebuild. If you spam save on a file (or even just hit it twice accidentally before it finishes) multiple builds get kicked off at the same time and they conflict. Sometimes it seems to catch it, sometimes it crashes fully.

1. I think we should revisit and speed this up. I suspect it's mostly pagefind so maybe we can add an option to skip that if a yaml file wasn't changed?
2. I think we may want to add a better catch or prevent multiple runs at once or something.

[ggetz]

there's an issue with the local server rebuild. If you spam save on a file (or even just hit it twice accidentally before it finishes) multiple builds get kicked off at the same time and they conflict. Sometimes it seems to catch it, sometimes it crashes fully.

I'll take a look and see if we can make a small change here to prevent the crash.

I do agree that outside of this PR, we can take a pass and see if there's something we can do to speed up the overall build step.

[ggetz]

@jjspace Updated!

• Added a small timeout to prevent multiple rebuilds when saving in quick succession
• Cleaned up error handling so that if the build function does throw, such as when missing a required property in sandcastle.yml, it is logged but does not stop the watcher or server.

[jjspace]

LGTM, may want further adjustments in the future but this seems fine for now