Provide a scriptable way to determine when VS finished loading the Intellisense

## Summary
When automating builds inside VS (in troubleshooting scenarios) it is important to let the Intellisense fully load before building. Yes, the status bar in VS says Ready much earlier, but there is enough empirical evidence to suggest otherwise when talking about big solutions with SDK and Legacy style projects in the mix.

It would be extremely useful to have a scriptable way to determine that Intellisense has been fully loaded. It could be through the `DTE2` interface or by checking the file system or anything at all as long as it can be done non interactively.

Since we are talking about special scenarios installing a dedicated VS extensions is also an option.

## User Impact
Regular devs do not care about it, this is for those who need to troubleshoot poor build performance.