Added directive for intellisense to find sst module in infra files

[NicoPowers]

When using VSCode or similar IDE's with Intellisense, they have trouble finding sst inside the infra folder and there is no way to import sst as a module.

By placing /// <reference path="../.sst/platform/config.d.ts" /> at the top of the index.ts in the infra folder, this allows intellisense find sst for those files.

[Paliago]

I can confirm this solves it for me as well.

[jayair]

Can you open an issue for this, I'll ask the team to test it.

[thdxr]

we're in the middle of switching repos (unfortunately github is making this quite messy) - going to close this PR for now feel free to open again once we've moved over to sst/sst