Add support for engine_files lastmod metadata

Apropos curiousdannii/parchment#177

Author: dfabulich

src/common/file/browser.ts

@@ -55,6 +55,10 @@ async function fetch_resource_inner(options: DownloadOptions, path: string, prog
     let url: URL | string
     try {
         url = new URL(path, lib_path)
+        const lastmod = options.engine_files?.[path]
+        if (lastmod) {
+            url.searchParams.set("lastmod", lastmod)
+        }
     }
     catch {
         url = lib_path + path

src/common/file/interface.ts

@@ -25,4 +25,5 @@ export interface DownloadOptions {
     /** Use the file proxy; if disabled may mean that some files can't be loaded */
     // We could just say to exclude proxy_url instead?
     use_proxy?: boolean | number,
-}
+    engine_files?: {[key: string]: string},
+}