fix: Add missing Symbol.observable typings to build output

.changeset/fresh-tips-yell.md

@@ -0,0 +1,5 @@
+---
+'wonka': patch
+---
+
+Add missing `Symbol.observable` global declaration back to typings.

src/helpers.ts

@@ -1,11 +1,5 @@
 import { TalkbackFn, TeardownFn, Start, Push, SignalKind } from './types';
 
-declare global {
-  interface SymbolConstructor {
-    readonly observable: symbol;
-  }
-}
-
 /** Placeholder {@link TeardownFn | teardown functions} that's a no-op.
  * @see {@link TeardownFn} for the definition and usage of teardowns.
  * @internal

src/observable.ts

@@ -1,6 +1,14 @@
 import { Source, SignalKind, TalkbackKind } from './types';
 import { push, start, talkbackPlaceholder, observableSymbol } from './helpers';
 
+// NOTE: This must be placed in an exported file for `rollup-plugin-dts`
+// to include it in output typings files
+declare global {
+  interface SymbolConstructor {
+    readonly observable: symbol;
+  }
+}
+
 /** A definition of the ES Observable Subscription type that is returned by
  * {@link Observable.subscribe}
  *