Add a workspace file to the template to trigger main.dart to open in the code editor (#7464)

packages/flutter_tools/templates/create/.idea/workspace.xml.tmpl

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="FileEditorManager">
+    <leaf>
+      <file leaf-file-name="main.dart" pinned="false" current-in-tab="true">
+        <entry file="file://$PROJECT_DIR$/lib/main.dart">
+          <provider selected="true" editor-type-id="text-editor">
+            <state relative-caret-position="0">
+              <caret line="0" column="0" lean-forward="false" selection-start-line="0" selection-start-column="0" selection-end-line="0" selection-end-column="0" />
+            </state>
+          </provider>
+        </entry>
+      </file>
+    </leaf>
+  </component>
+  <component name="ToolWindowManager">
+    <editor active="true" />
+    <layout>
+      <window_info id="Project" active="false" anchor="left" auto_hide="false" internal_type="DOCKED" type="DOCKED" visible="true" show_stripe_button="true" weight="0.25" sideWeight="0.5" order="0" side_tool="false" content_ui="combo" />
+    </layout>
+  </component>
+</project>