Clean up and tweak ESLint rules.

Also, refactor ESLint and stylelint config files; now everything will work from our root dir.

build/.eslintrc.json

@@ -0,0 +1,20 @@
+{
+  "env": {
+    "browser": false,
+    "node": true
+  },
+  "parserOptions": {
+    "sourceType": "script"
+  },
+  "extends": "../.eslintrc.json",
+  "rules": {
+    "consistent-return": "off",
+    "func-style": "off",
+    "no-console": "off",
+    "no-magic-numbers": "off",
+    "no-process-env": "off",
+    "no-process-exit": "off",
+    "no-sync": "off",
+    "spaced-comment": "off"
+  }
+}