vscode: add "source" field to executeCommand metric (#607)

When a vscode command is executed it can be executed
from different places. The 'source' field will help
to identify what caused the command to execute.

Signed-off-by: nkomonen <[email protected]>

Author: nkomonen-amazon

telemetry/definitions/vscodeDefinitions.json

@@ -35,6 +35,7 @@
             "passive": true,
             "metadata": [
                 { "type": "command" },
+                { "type": "source" },
                 { "type": "debounceCount" },
                 { "type": "duration" },
                 { "type": "result" },