Fix rojo plugin install by adding Vfs::exists

[kennethloeffler]

In #1159, we added a new helper function for loading a project file:

rojo/src/project.rs

Lines 291 to 307 in 2b47861

	pub(crate) fn load_initial_project(vfs: &Vfs, path: &Path) -> Result<Self, ProjectError> {
	if Self::is_project_file(path) {
	Self::load_exact(vfs, path, None)
	} else {
	// Check for default projects.
	for default_project_name in DEFAULT_PROJECT_NAMES {
	let project_path = path.join(default_project_name);
	if project_path.exists() {
	return Self::load_exact(vfs, &project_path, None);
	}
	}
	Err(Error::NoProjectFound {
	path: path.to_path_buf(),
	}
	.into())
	}
	}

At line 298, we use Path::exists. This doesn't work at all when we're using the in memory file system, or in general, because the virtual file system need not have any correspondence to the host's file system. This causes the rojo plugin install command to always fail, because it uses the in memory file system to build the plugin RBXM.

This PR fixes this problem by adding a Vfs::exists method that is equivalent to std::fs::exists, and using it in load_initial_project. It also adds a smoke test to the plugin install command to ensure the plugin initializes correctly.

I also moved the create_dir and create_dir_all memofs changelog entry under the unreleased header, because that change has not made it into a memofs release yet, and we missed this during review.