Phase 8.3: Fix spawn to use bux_task_spawn for non-async functions

- Added exprSpawnAsync flag to AST ekSpawn
- Sema checks if callee is async and sets the flag
- HIR hSpawn carries spawnAsync flag
- C backend uses bux_task_spawn for non-async (OS threads) and bux_async_spawn for async (coroutines)
- Added concurrency example to test suite

src/hir.nim

@@ -118,6 +118,7 @@ type
     of hSpawn:
       spawnCallee*: string
       spawnArgs*: seq[HirNode]
+      spawnAsync*: bool
     of hEmit:
       emitCode*: string
     of hDynRef: