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_lower.nim

@@ -832,6 +832,7 @@ proc lowerExpr(ctx: var LowerCtx, expr: Expr): HirNode =
     for arg in expr.exprSpawnArgs:
       args.add(ctx.lowerExpr(arg))
     return HirNode(kind: hSpawn, spawnCallee: calleeName, spawnArgs: args,
+                   spawnAsync: expr.exprSpawnAsync,
                    typ: makePointer(makeVoid()), loc: loc)
 
   of ekAwait: