feat: support additional bun compile args and --bytecode compatibility

[ramonmalcolm10]

Problem

1. There's no way to pass additional flags (e.g. --minify, --sourcemap, --bytecode) to bun build --compile during the adapter's
   compile step.
2. The sveltekit server entry uses top-level await, which causes bun build --compile --bytecode to fail with:
   error: "await" can only be used inside an "async" function
3. The gracefulShutdown handler runs twice on Ctrl+C because both SIGINT and SIGTERM fire, producing duplicate logs:
   Stopping server...
   Stopping server...
   Stopped server
   Stopped server

Proposed Changes

• All packages (sveltekit, nuxt, tanstack): Add a bunArgs option that passes additional flags to bun build --compile
• sveltekit: Wrap the server entry in an async IIFE to eliminate top-level await for --bytecode compatibility
• sveltekit: Add a guard to gracefulShutdown to prevent duplicate execution

Usage

  // svelte.config.js
  adapter({
    bunArgs: ["--minify", "--sourcemap", "--bytecode"]
  })