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/ast.nim

@@ -207,6 +207,7 @@ type
     of ekSpawn:
       exprSpawnCallee*: Expr
       exprSpawnArgs*: seq[Expr]
+      exprSpawnAsync*: bool
     of ekAwait:
       exprAwaitOperand*: Expr
     of ekBlock: