Linter reform, add Prettier

Overall, goal is to make sure the linter is an aid, not a barrier, to
contribution, by ensuring all linting rules serve one of the following
purposes:

1. Catch likely coding mistakes
2. Enforce major, uncontroversial best practices

3. Enforce opinionated/arbitrary style/formatting rules, **with autofixing**.
   In particular, makes the following changes:

- Removes overly prescriptive eslint rules
- Uses presets for eslint and all plugins; overrides some rules to
  downgrade from `error` to `warn` but otherwise removes redundant rules
  from rc
- Adds Prettier to development environment; configures eslint and
  stylelint integration with Prettier
- Removes eslint and stylelint rules that conflict or are redundant with Prettier
- Adds new `fixlint.prettier` and `lint.prettier` tasks that apply and
  verify Prettier formatting for file types that don’t have Prettier
  integration in their linters; these are included in the top level
  `fixlint` and `lint` tasks

Note that this commit does _not_ actually modify source files to conform to
updated formatting; that will be in a separate commit to make upgrading open
branches easier.

.eslintignore

@@ -0,0 +1 @@
+config/lintignore