Add trailing commas to multi-line objects and arrays

[osteele]

Enable the comma-dangle eslint rule.

Add yarn format to fix eslint issues.

Fix a handful of comma-dangle issues.

Note: a couple of concessions are made, in order to prevent a large number of changes that woud likely cause merge conflicts with outstanding branches:

1. comma-dangle is a warning, not an error
2. files in src aren't fixed

Required

Changes must conform to these requirements:

• [x] yarn test passes. All new and existing tests pass.
• [x] yarn lint passes. All new code follows the code style of this project.

Aspirational

We don't yet require these, but they are nice to have:

• [x] New code is covered by new or existing tests.
• [x] Changed code is covered by new or existing tests.

[osteele]

I experimented with adopting all of AirBnB style, but it required either a large number of exceptions (work in progress here), or a large number of code changes (log here — note that that's with all the exceptions in the previous link). Later we might want to adopt at least semi and quotes, when there's not as much work in branches.