reorg: remove dispatch.ts, move signals, factor out web utils (#4316)

- moves signal definition from "cli/js/process.ts" to "cli/js/signals.ts"
- removes "cli/js/dispatch.ts"
- removes "cli/js/types.ts"
- moves web specific utilities to "cli/js/web/util.ts"

cli/js/web/fetch.ts

@@ -1,10 +1,6 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
-import {
-  assert,
-  createResolvable,
-  notImplemented,
-  isTypedArray
-} from "../util.ts";
+import { assert, createResolvable, notImplemented } from "../util.ts";
+import { isTypedArray } from "./util.ts";
 import * as domTypes from "./dom_types.ts";
 import { TextDecoder, TextEncoder } from "./text_encoding.ts";
 import { DenoBlob, bytesSymbol as blobBytesSymbol } from "./blob.ts";