WIP: pls extract the other stuff into own commits

node signaling is done by providing a `progress.Root` to every node,
dispatching events to it when the node changes. the root is connected to
an observer to construct a UI out of it. this commit adds two built in
observers:

`attachToScreen` binds a root to a TTY screen (via the log.Widget API).
the primary use of this is to implement the top level `progress.start`.

`encodeStream` converts these events into a `ReadableStream`. by
batching events together, the stream contents remain small, that way the
code that constructs progress nodes do not have to worry about calling
many setters at once, it gets debounced be the serializer. a server may:

    const root = new progress.Root();
    doActionWithProgress(root satisfies progress.Ref)
      .then(
        (result) => root.end({ success: true, result }),
        (error) => root.end({ success: false, error })
      );
    if (req.headers.get("Accept").includes(progress.contentType))
      return new Response(progress.encodeStream(root), {
        headers: { 'Content-Type': progress.contentType },
      });
    // to support non-streaming clients
    const result = await root.once("end");
    return result.success ? ... : ...;

and `decodeStream` on the client:

    const output = document.getElementById("output");
    const res = await fetch(...);
    if (!res.ok) throw ...;
    const root = progress.decodeStream(res.body);
    output.innerHTML = progress.formatHtml(root.active);
    root.on("change", () => {
      output.innerHTML = progress.formatHtml(root.active);
    });
    const result = await root.once("end");
    output.innerText = JSON.stringify(result);

this feature is something that my work is interested in using. while
`progress.ts` will not be the home of a React hook -- it's trivial to
implement

the library overall gains extra features:

- a typed event emitter (`Events.ts`) compatible with the browser
- cancelable promises in `async.ts`
- more type helpers in `ts.ts`

@@ -6,3 +5,1 @@
- *     // Extra types are in a `declare namespace`
- *     const opts: Lru.Options = { capacity: 2 };
- *     const lru = new Lru(opts);
+ * ```

@@ -405,0 +1328,4 @@
+
+  estimatedTimeNonNull: boolean;
+
+  futureProofingPacket: boolean;

@@ -4,20 +4,47 @@
 
 ### breaking

@@ -201,13 +201,16 @@ export function mergeStyles(code: string): string {
  * turned into their original names.
  */
 export function debugAnsi(text: string): string {
+  // return JSON.stringify(text);