Provide Run Selection button in Editor Title Menu

This commit adds a editor title menu button for the
`powershell.RunSelection` command in the primary navigation group, and
the `run` icon from the https://github.com/microsoft/vscode-icons repo.
This shows the button in the visible part of the menu when a PowerShell
file is open.

Author: jpogran

package.json

@@ -164,7 +164,11 @@
       {
         "command": "PowerShell.RunSelection",
         "title": "Run Selection",
-        "category": "PowerShell"
+        "category": "PowerShell",
+        "icon": {
+          "light": "resources/light/play.svg",
+          "dark": "resources/dark/play.svg"
+        }
       },
       {
         "command": "PowerShell.RestartSession",
@@ -260,6 +264,13 @@
           "group": "2_powershell"
         }
       ],
+      "editor/title": [
+        {
+          "when": "resourceLangId == powershell",
+          "command": "PowerShell.RunSelection",
+          "group": "navigation@100"
+        }
+      ],
       "editor/title/context": [
         {
           "when": "resourceFilename =~ /\\.tests\\.ps1$/i",