How to upgrade TypeScript.

The files in this directory are mostly from the TypeScript repository. We currently (unfortunately) have a rather manual process for upgrading TypeScript. It works like this currently:

1. Checkout typescript repo in a separate directory.
2. Copy typescript.js into Deno repo.
3. Copy d.ts files into dts directory.

So that might look something like this:

git clone https://github.com/microsoft/TypeScript.git
cd typescript
git checkout v3.9.7
rsync lib/typescript.js ~/src/deno/cli/tsc/00_typescript.js
rsync --exclude=protocol.d.ts --exclude=tsserverlibrary.d.ts --exclude=typescriptServices.d.ts lib/*.d.ts ~/src/deno/cli/dts/