Rename incorrectly named .d.ts files

These files do not describe existing JS types and instead define their
own. Usage of .d.ts extension prevents these types from being emitted
when using emitDeclarationOnly, which affects generation of typings for
mod development.

src/js/mods/mod_metadata.ts

@@ -33,6 +33,6 @@ export interface ModInfo {
     mod: Mod;
 }
 
-export interface FrozenModMetadata extends Readonly<ModMetadata> {
+export interface FrozenModMetadata extends Readonly<Omit<ModMetadata, "authors">> {
     authors: ReadonlyArray<Readonly<ModAuthor>>;
 }