Merge deno completions update from #9676

(Can't cherry-pick because GitHub tricked me into rebasing instead of
squashing.)

share/completions/deno.fish

@@ -1 +1,19 @@
 deno completions fish | source
+
+# complete deno task
+set searchForDenoFilesCode '
+// order matters
+const denoFile = ["deno.json", "deno.jsonc", "package.json"];
+for (const file of denoFile) {
+  try {
+    Deno.statSync(file);
+    // file exists
+    const props = file === "package.json" ? "scripts" : "tasks";
+    console.log(
+      Object.keys(JSON.parse(Deno.readTextFileSync(file))[props]).join("\n"),
+    );
+    break;
+  } catch {}
+}
+'
+complete -f -c deno -n "__fish_seen_subcommand_from task" -n "__fish_is_nth_token 2" -a "(deno eval '$searchForDenoFilesCode')"