Fix avoid upstream typechecking issue

Running `npm install` with the JupyterLab 2.0.0 dependencies resulted in
an error.

```
node_modules/@jupyterlab/ui-components/lib/icon/labicon.d.ts:3:8 - error TS1259: Module '"/home/erik/dev/contrib/jupyterhub/jupyter-server-proxy/jupyterlab-server-proxy/node_modules/@types/react/index"' can only be default-imported using the 'esModuleInterop' flag

3 import React from 'react';
         ~~~~~

  node_modules/@types/react/index.d.ts:62:1
    62 export = React;
       ~~~~~~~~~~~~~~~
    This module is declared with using 'export =', and can only be used with a default import when using the 'esModuleInterop' flag.
```

Adding an option that influenced the typechecking, but not the actual
execution, made it go away. I learned it from this stackoverflow answer.

https://stackoverflow.com/a/56348146/2220152

Note that the option `allowSyntheticDefaultImports` is described like
this, which makes me safe that we won't influence actual execution but
only the type checking part.

> Allow default imports from modules with no default export. This does not
affect code emit, just typechecking.

Author: consideRatio

jupyterlab-server-proxy/tsconfig.json

@@ -1,5 +1,6 @@
 {
   "compilerOptions": {
+    "allowSyntheticDefaultImports": true,
     "declaration": true,
     "lib": ["es2015", "dom"],
     "module": "commonjs",