prettier: check in .prettierrc.json

Summary:
Justifications:

  - We use `singleQuote: true` is for consistency with Google’s
    JavaScript style guide.
  - We don’t use `trailingComma: "all"` because that’s incompatible with
    some compilers in our toolchain; `trailingComma: "es5"` gets us most
    of the way there.
  - Other options are due to prevalence/preference.

Test Plan:
Running Prettier on the `.prettierrc.json` file itself succeeds, so the
file is syntactically and semantically valid.

wchargin-branch: prettierrc

.prettierrc.json

@@ -0,0 +1,6 @@
+{
+  "singleQuote": true,
+  "trailingComma": "es5",
+  "bracketSpacing": false,
+  "arrowParens": "always"
+}