Type checking in adapters

[benmccann]

Issue

The adapters don't have TypeScript. This leads two two big issues.

1. we introduce bugs which would be caught with type checking https://github.com/sveltejs/kit/pull/600/files#r600142752
2. VS Code is painful to use because everything is red squigglies

Possible solutions

microsoft/TypeScript#33079 (comment)