Debug serially in the "Debugging tests with VS Code" recipe

[troysandal]

Debug serially to avoid confusing execution paths in async code, e.g. swapping from one test's execution context to the next.

[novemberborn]

Thanks for your PR @troysandal! Those execution paths may be the cause of test failures, though. Rather than suggesting this as the default invocation, perhaps you could repeat the configurations section with the --serial flag included, and a brief explanation of why this might be useful?

[troysandal]

@novemberborn great point Mark, I added a new section under Debug that addresses this, thank you.

[novemberborn]

Thanks @troysandal. I've made a few more changes, please let me know if you're happy with them.

[troysandal]

Changes look great @novemberborn and @sindresorhus thank you.