Start testing TypeScript output on CI

This commit starts to add some simple tests for our TypeScript output of
the wasm-bindgen CLI, currently just running `tsc` to make sure syntax
looks good and types are emitted as expected. This'll hopefully be able
to get expanded over time with bug reports as they come in as well as
ensure that we don't regress anything in egregious manners!

Closes #922

crates/typescript-tests/index.ts

@@ -0,0 +1,11 @@
+import * as wbg from './pkg/typescript_tests';
+import * as wasm from './pkg/typescript_tests_bg';
+
+const a1: (a: string) => void = wbg.greet;
+const a2: (a: number, b: number) => void = wasm.greet;
+const a3: WebAssembly.Memory = wasm.memory;
+
+const c = new wbg.A();
+wbg.A.other();
+c.foo();
+c.free();