Output multi-element TOML arrays on newlines

[sdboyer]

What does this do / why do we need it?

Our TOML integration currently writes out multi-element arrays on a single line. This is not great for reading, and also not great for comprehensible diffs and erroneous conflicts. Our TOML lib just updated to add support for writing each item on a newline, and this PR incorporates the new logic.

What should your reviewer look out for in this PR?

i don't think we need any more exhaustive testing on this - seems like what we have should be sufficient.

[carolynvs]

Yay! 🎉 This is much easier to read.

[vektah]

It would be nice if we had trailing commas on the final line to reduce diff noise:

packages = [
   ".",
   "subp"
]

vs

packages = [
   ".",
   "subp",
]

Also, is there any way to add a dep version requirement to the manifest? similar to npm's engines so we can prompt other devs working with our codebases that they need to update dep?

[sdboyer]

@vektah new issues for each of those, please 😊