Add troubleshooting guide and editorconfig

[splint-disk-8i]

Adds a troubleshooting section to BUILDING.md covering the most common build issues I ran into while setting up the project. Also adds an .editorconfig so editors that support it automatically pick up the project's indentation style.

Changes:

• BUILDING.md: new troubleshooting section with fixes for Xcode setup, Swift version mismatches, permission errors, network issues during integration tests, kernel install timeouts, and the known vmnet bug
• .editorconfig: tabs for Makefiles, 4-space indent for Swift/Shell, 2-space for YAML/JSON/TOML

[saehejkang]

This is super helpful and thank you for starting on it! Take a look at #773, as it relates to the changes you made here.

Feel free to work on any other points in that issue, but due to scope, I think this PR is good as is and should serve as the initial setup of the section itself, with a couple of solutions (like you have). This also might live better in its own file and can instead be linked to in this file.

[splint-disk-8i]

Thanks! Just looked at #773 — makes sense to keep a dedicated troubleshooting doc. Want me to split it into a separate file and link it from BUILDING.md in this PR, or should I do that in a follow-up?

[saehejkang]

I think the changes should be done in this PR. The idea is to follow the first step from the issue, but start on a much smaller scale, since this will serve as the baseline to follow in the future. Also, there will be a lot more issues to add and we don't the Building.md file to become too large.

What do you think about the steps below for a first iteration?

1. Create the new troubleshooting doc and add a link to it in Building.md.
2. For any issues you've already identified, cross-check them with related open or closed issues and link those in the new doc.

[splint-disk-8i]

Good idea — I moved the troubleshooting content into its own TROUBLESHOOTING.md and added a link from BUILDING.md. Also cross-referenced a few related issues, where I could find them. Let me know if this is what you had in mind.