fix: Don’t require getCollection() filter to be a type guard

Commit dabce6b (#5970) broke the use
of a plain boolean filter.  Add an overload similar to TypeScript’s
Array#filter overload:

https://github.com/microsoft/TypeScript/blob/v4.9.4/lib/lib.es5.d.ts#L1442-L1453

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

packages/astro/src/content/template/types.d.ts

@@ -48,6 +48,10 @@ declare module 'astro:content' {
 		collection: C,
 		filter?: (entry: CollectionEntry<C>) => entry is E
 	): Promise<E[]>;
+	export function getCollection<C extends keyof typeof entryMap>(
+		collection: C,
+		filter?: (data: CollectionEntry<C>) => unknown
+	): Promise<CollectionEntry<C>[]>;
 
 	type InferEntrySchema<C extends keyof typeof entryMap> = import('astro/zod').infer<
 		Required<ContentConfig['collections'][C]>['schema']