Fix unintentional missing type data in MarkdownInstance (#3397)

* Update MarkdownInstance type

The return of the `default` function includes the same `frontmatter`
data as the parent object, merged with the `astro` data. The inclusion
of that frontmatter type was previously not recognized by TS, and fell
back to a `Record<string, any>`. This change persists the more accurate
type, as the runtime code does.

* fixup! Update MarkdownInstance type

(This change is what I'd personally do, but I don't really know how you
expect people to use `MarkdownContent` in practice, or if there is some
deeper benefit you wish to exploit by leaving it as an interface instead
of a type.

.changeset/eleven-planes-deliver.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+MarkdownInstance: Persist frontmatter type into the return of `.default()`

packages/astro/src/@types/astro.ts

@@ -760,7 +760,7 @@ export interface MarkdownInstance<T extends Record<string, any>> {
 	getHeaders(): Promise<MarkdownHeader[]>;
 	default: () => Promise<{
 		metadata: MarkdownMetadata;
-		frontmatter: MarkdownContent;
+		frontmatter: MarkdownContent<T>;
 		$$metadata: Metadata;
 		default: AstroComponentFactory;
 	}>;
@@ -817,10 +817,9 @@ export interface MarkdownParserResponse extends MarkdownRenderingResult {
  * The `content` prop given to a Layout
  * https://docs.astro.build/guides/markdown-content/#markdown-layouts
  */
-export interface MarkdownContent {
+export type MarkdownContent<T extends Record<string, any> = Record<string, any>> = T & {
-	[key: string]: any;
 	astro: MarkdownMetadata;
-}
+};
 
 /**
  * paginate() Options