Add VS Code launch and task files

This enables building PowerShell through VS Code. The build task
launches the installed version of PowerShell and runs
`Start-PSBuild` (after importing the module).

This enables a `launch` debug task to immediately debug the `powershell`
process, without having to attach to an external process.

The defaults of `justMyCode` and `stopAtEntry` have been reversed, so
that all code is debugged, and the process is stopped at entry for
easier debugging.

Note that an interactive PowerShell process requires access to `stdin`,
which the debug console does not provide, so once it requests access,
`System.Console` will throw a (correct) exception.

.vscode/launch.json

@@ -0,0 +1,16 @@
+{
+	"version": "0.2.0",
+	"configurations": [
+		{
+			"name": "powershell",
+			"type": "coreclr",
+			"request": "launch",
+			"justMyCode": false,
+			"stopAtEntry": true,
+			"program": "${workspaceRoot}/bin/powershell",
+			"args": [ ],
+			"preLaunchTask": "build",
+			"cwd": "${workspaceRoot}"
+		}
+	]
+}