Rename extension.js to core.js to better reflect its use

ext/bg/background.html

@@ -18,7 +18,7 @@
         <script src="/mixed/lib/jszip.min.js"></script>
         <script src="/mixed/lib/wanakana.min.js"></script>
 
-        <script src="/mixed/js/extension.js"></script>
+        <script src="/mixed/js/core.js"></script>
 
         <script src="/bg/js/anki.js"></script>
         <script src="/bg/js/mecab.js"></script>

ext/bg/context.html

@@ -178,7 +178,7 @@
             </a>
         </div>
 
-        <script src="/mixed/js/extension.js"></script>
+        <script src="/mixed/js/core.js"></script>
 
         <script src="/bg/js/api.js"></script>
         <script src="/bg/js/options.js"></script>

ext/bg/search.html

@@ -60,7 +60,7 @@
         <script src="/mixed/lib/handlebars.min.js"></script>
         <script src="/mixed/lib/wanakana.min.js"></script>
 
-        <script src="/mixed/js/extension.js"></script>
+        <script src="/mixed/js/core.js"></script>
 
         <script src="/bg/js/dictionary.js"></script>
         <script src="/bg/js/handlebars.js"></script>

ext/bg/settings-popup-preview.html

@@ -117,7 +117,7 @@
             </div>
         </div></div></div>
 
-        <script src="/mixed/js/extension.js"></script>
+        <script src="/mixed/js/core.js"></script>
         <script src="/fg/js/api.js"></script>
         <script src="/fg/js/document.js"></script>
         <script src="/fg/js/frontend-api-receiver.js"></script>

ext/bg/settings.html

@@ -864,7 +864,7 @@
         <script src="/mixed/lib/handlebars.min.js"></script>
         <script src="/mixed/lib/wanakana.min.js"></script>
 
-        <script src="/mixed/js/extension.js"></script>
+        <script src="/mixed/js/core.js"></script>
         <script src="/mixed/js/japanese.js"></script>
 
         <script src="/bg/js/anki.js"></script>

ext/fg/float.html

@@ -31,7 +31,7 @@
             </div>
         </div>
 
-        <script src="/mixed/js/extension.js"></script>
+        <script src="/mixed/js/core.js"></script>
 
         <script src="/fg/js/api.js"></script>
         <script src="/fg/js/util.js"></script>

ext/manifest.json

@@ -18,7 +18,7 @@
     "content_scripts": [{
         "matches": ["http://*/*", "https://*/*", "file://*/*"],
         "js": [
-            "mixed/js/extension.js",
+            "mixed/js/core.js",
             "fg/js/api.js",
             "fg/js/document.js",
             "fg/js/frontend-api-receiver.js",