Enforce newline at end of files

Trailing newlines are a common idiom in
programming, and enabling this rule will save
reviewers nitpicking or overlooking missing
trailing newlines.

ESLint also supports the `--fix` option to
automate fixing some of the problems reported by
this rule.

There are no current files in the codebase that
require fixing to add trailing newlines as
verified by `npm run lint`.

.eslintrc.json

@@ -12,6 +12,10 @@
         "sourceType": "module"
     },
     "rules": {
+        "eol-last": [
+            "error",
+            "always"
+        ],
         "indent": [
             "error",
             2