This upgrades the version of TypeScript Tsickle uses as a
`devDependency` and a `peerDependency`.

Support for Private Fields
--------------------------
Tsickle mostly ignores private fields, however it no longer warns when
not generating externs for them. Externs are not generated because the
fields no not exist on the class when downleveled.

Support for `export * as ns` Syntax
-----------------------------------
Tsickle compiles:

```ts
export * as ns from './namespace';
```

to

```ts
var tsickle_module_1_ = goog.require('project.namespace');
exports.ns = tsickle_module_1_;
```

New `tslib` Functions
---------------------
Private fields require two new functions in `tslib`:
`__classPrivateFieldGet` and `__classPrivateFieldSet`. Both of these
were added to Tsickle's version of `tslib.js` along with Closure type
annotations.

Other Changes
-------------
* The `import_export_typedef_conflict` test was removed because that
  code is no longer valid in TypeScript 3.8, and produces a compile
  error.
* `tslib@1.11.0` was added as a `devDependency`. Before it was being
  pulled in as a transitive dependency, but the packages pulling it in
  depend on `1.10`. To upgrade to TypeScript 3.8 Tsickle needs the latest
  version of `tslib`.