Replace public peek method with friend-style symbol

Peeking a value in contexts other than internal parts of the reactivity
system itself tends very strongly to produce bugs, because it decouples
consumers from the root state. (It is very, very tempting to wire your
own caching on with a "peek", rather than using caching tools composed
out of the core primitives, or to "be smarter" than the signal system.)

For an interesting background discussion from the history of Glimmer's
similar tag/signals system, see [here][github].

[github]: emberjs/rfcs#656 (comment)

This doesn't *force* us to keep that, but it sets a nice precedent and
means if we *do* introduce `peek()` as public behavior, we'll have a
clear indicator that we need to make the choice explicitly.

src/signal.ts

@@ -7,6 +7,12 @@ function baseEquality<T>(oldValue: T, newValue: T) {
 }
 
 export class Signal<T = unknown> {
+/**
+  @private This is available for internal "friend" APIs to use, but it is *not*
+    legal to use by consumers.
+ */
+export const Peek = Symbol('Peek');
+
   #value: T;
   protected isEqual: Equality<T>;
   protected tag: Tag;
@@ -27,7 +33,12 @@ export class Signal<T = unknown> {
     return this.#value;
   }
 
-  peek() {
+  // Expressly *not* part of the public API: peeking a value in contexts other than internal parts
+  // of the reactivity system itself tends very strongly to produce bugs, because it decouples
+  // consumers from the root state. (It is very, very tempting to wire your own caching on with a
+  // "peek", rather than using caching tools composed out of the core primitives, or to "be smarter"
+  // than the signal system.)
+  [Peek](): T {
     return this.#value;
   }
 

tests/signal.spec.ts

@@ -1,5 +1,5 @@
 import { expect, describe, test, vi } from 'vitest';
-import { createSignal } from '../src/signal';
+import { createSignal, Peek } from '../src/signal';
 import { createEffect } from '../src/effect';
 import isEqual from 'lodash/isEqual';
 
@@ -50,7 +50,7 @@ describe('Signal', () => {
     let foo = createSignal('foo');
 
     let spy = vi.fn(() => {
-      foo.peek();
+      foo[Peek]();
     });
 
     createEffect(spy);