Further speed improvements

- Store Emscripten cache inside node_modules/.em_cache. Docker image ships without LTO libs, so Emscripten has to rebuild stdlibs on every build otherwise.
 - Merge webp_enc + webp_dec build scripts. Core libwebp library is same in both cases, so there's no point in storing and building two copies of it.

codecs/webp/dec/webp_dec.d.ts

@@ -0,0 +1,13 @@
+interface RawImage {
+  buffer: Uint8Array;
+  width: number;
+  height: number;
+}
+
+interface WebPModule extends EmscriptenWasm.Module {
+  decode(data: BufferSource): RawImage;
+  free_result(): void;
+}
+
+export default function(opts: EmscriptenWasm.ModuleOpts): WebPModule;
+