fix: use closure-typed HandlerAsync for middleware in Nim 2.0

Nim 2.0 doesn't implicitly convert top-level procs to closures.
Prologue's use() expects HandlerAsync (closure type), so declaring
the middleware as a let variable typed HandlerAsync forces the proc
to be a closure.

Author: BennyFranciscus

frameworks/prologue/src/server.nim

@@ -242,11 +242,11 @@ let settings = newSettings(
 
 var app = newApp(settings = settings)
 
-proc addServerHeader(ctx: Context) {.async.} =
+let serverHeaderMiddleware: HandlerAsync = proc(ctx: Context) {.async.} =
   ctx.response.setHeader("Server", "prologue")
   await switch(ctx)
 
-app.use(addServerHeader)
+app.use(serverHeaderMiddleware)
 
 app.addRoute("/pipeline", pipelineHandler, HttpGet)
 app.addRoute("/baseline11", baseline11Handler, HttpGet)