Add (or document) option to prevent scanning of composer packages

[OldStarchy]

I suspect adrhumphreys/vscode-silverstripe#23 is caused by sanchez scanning composer for additional snippets and configuration.

I've had a look through the code and haven't been able to work out why.

The project I'm working in (the one I mentioned here) has 2436 packages (according to yarn list | wc -l) and about 100 composer packages (according to tree -L 2 -d vendor) and it takes about 60 seconds for the silverstripe extension to start.

Deleting the node_modules doesn't change the start up time at all, but deleting the vendor folder reduced it down to 12 seconds consistently.

[lerni]

Just confirming, that performance implications on start/scan are huge.