Update code for the review comments

* Make decode module return value `ImageData`
* Fix global definition of ImageData
* Use concrete Encoder types for encode functions
* Use ArrayBufferView in FileLike instead of using a similar type
* Throw error when the `encode` functions
return null
* Use generic types for WorkerPool
* Fix `encode` function typing
in `index.ts`
* Remove ts-ignore for web-streams-polyfill
and handle nulls for TransformStream
* Fix rollup entry point (now we need to have
`index.ts` instead of `index.js`)

libsquoosh/rollup.config.js

@@ -10,7 +10,7 @@ import { builtinModules } from 'module';
 
 /** @type {import('rollup').RollupOptions} */
 export default {
-  input: 'src/index.js',
+  input: 'src/index.ts',
   output: {
     dir: 'build',
     format: 'cjs',