Protect against syncing and building the wrong project types

[Dekkonot]

Closes #736.

Prevents projects with a data model as a root from being built as a model file and vice versa. Also prevents syncing model files altogether.

This should mitigate a few of the more common problems with building Rojo projects. I'm not sure whether I should revert #691 or not. The check it adds is made moot with this PR, but it could be a good failsafe. Open to suggestion.

This doesn't impact the VSCode extension at all because it determines the file type to build as using a similar heuristic as this PR and listens to errors during serving to display to the user.

[Dekkonot]

You know, that's on me I didn't think to test whether our own tests would break if we changed this, I will fix it soon:tm:. It is currently 80 degrees and my laptop is hot though, so it'll have to wait.

[kennethloeffler]

The fact that tests were relying on this behavior is pretty funny! 😂

While technically a breaking change, I doubt anyone ever actually intended to do this, or got a working result out of it... so it's probably fine?

[boatbomber]

Regarding #691, I think leaving that protection in the plugin is still valuable both as a failsafe and as protection against serves from otherwise compatible <=7.3.0 servers.

[Dekkonot]

We can revisit tests later, since I'm not sure on the names of some of these. I want to keep the diff as relevant as possible though, since this turned into a bigger patch than anticipated.

[Dekkonot]

..Apologies for whatever the hell notification that just sent you, Ken. Github's UI isn't intuitive for this.