Add basic ESLint configuration for formatting

This doesn't enable any of ESLint's actual possible-issue linting,
but just style normalization based on the Prettier configuration (but without line length limits).

.gitignore

@@ -34,3 +34,5 @@ notification.mp3
 /test/stderr.txt
 /cache.json*
 /config_states/
+/node_modules
+/package-lock.json