chore: Small tweaks to the eslint config files (follow-up for #167)

.eslintignore

@@ -0,0 +1,2 @@
+coverage/
+dist/

.eslintrc

@@ -8,10 +8,8 @@
   },
 
   "env": {
-    "browser": true,
     "es6": true,
     "node": true,
-    "webextensions": true,
   },
 
   "globals": {

src/.eslintrc

@@ -1,9 +1,11 @@
 {
   "env": {
+    "browser": true,
     "node": false,
+    "webextensions": true,
   },
   "globals": {
     // Allow the `module` global, but not the `require(…)` function
     "module": false,
   },
-}
+}

test/.eslintrc

@@ -1,6 +1,8 @@
 {
   "env": {
+    "browser": true,
     "mocha": true,
+    "webextensions": true,
   },
   "rules": {
     "max-nested-callbacks": ["warn", 6]