core.mdx: Add "unknown" value to JSMinifyOptions module (#251)

Hey folks! 👋

swc-project/swc#9026 added a new value to the
`module` property in `JSMinifyOptions`, but the type declaration in
`core.mdx` was not updated.

This change fixes that.

Please let me know if I missed anything!

pages/docs/usage/core.mdx

@@ -198,7 +198,7 @@ export interface JsMinifyOptions {
     ecma?: TerserEcmaVersion;
     keep_classnames?: boolean;
     keep_fnames?: boolean;
-    module?: boolean;
+    module?: boolean | "unknown";
     safari10?: boolean;
     toplevel?: boolean;
     sourceMap?: boolean;