add hasContext lifecycle function (#5690)

site/content/docs/03-run-time.md

@@ -178,6 +178,26 @@ Retrieves the context that belongs to the closest parent component with the spec
 </script>
 ```
 
+#### `hasContext`
+
+```js
+hasContext: boolean = hasContext(key: any)
+```
+
+---
+
+Checks whether a given `key` has been set in the context of a parent component. Must be called during component initialisation.
+
+```sv
+<script>
+	import { hasContext } from 'svelte';
+
+	if (hasContext('answer')) {
+		// do something
+	}
+</script>
+```
+
 #### `createEventDispatcher`
 
 ```js

src/runtime/internal/lifecycle.ts

@@ -54,6 +54,10 @@ export function getContext<T>(key): T {
 	return get_current_component().$$.context.get(key);
 }
 
+export function hasContext(key): boolean {
+	return get_current_component().$$.context.has(key);	
+}
+
 // TODO figure out if we still want to support
 // shorthand events, or if we want to implement
 // a real bubbling mechanism