core-api: add global definition of Symbol.observable

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/old-pigs-happen.md

@@ -0,0 +1,5 @@
+---
+'@backstage/core-api': patch
+---
+
+Add a global type definition for `Symbol.observable`, fix type checking in projects that didn't already have it defined.

packages/core-api/src/types.ts

@@ -42,6 +42,14 @@ export type Subscription = {
   readonly closed: boolean;
 };
 
+// Declares the global well-known Symbol.observable
+// We get the actual runtime polyfill from zen-observable
+declare global {
+  interface SymbolConstructor {
+    readonly observable: symbol;
+  }
+}
+
 /**
  * Observable sequence of values and errors, see TC39.
  *