Provide highlighting for translation comments.

Close #115.

Author: seanwu1105

src/test/assets/qml/translator-comments.qml

@@ -0,0 +1,8 @@
+Text {
+    id: txt1;
+    // This user interface string is only used here
+    //: The back of the object, not the front
+    //= qtn_foo_bar
+    //~ Context Not related to back-stepping
+    text: qsTr("Back");
+}

syntaxes/qml.tmLanguage.json

@@ -260,6 +260,23 @@
     },
     "comment": {
       "patterns": [
+        {
+          "begin": "(\\/\\/[~|=])\\s*([A-Za-z_$][\\w$.\\[\\]]*)",
+          "beginCaptures": {
+            "1": {
+              "name": "storage.type.class.qml.tr"
+            },
+            "2": {
+              "name": "variable.other.qml.tr"
+            }
+          },
+          "end": "$",
+          "patterns": [
+            {
+              "include": "#comment-contents"
+            }
+          ]
+        },
         {
           "begin": "(\\/\\/)",
           "beginCaptures": {