chore: avoid microtasks when flushing sync#15855

This has a significant impact on our benchmark suite because we avoid creating microtasks in a sync benchmark. We previously had this logic in the codebase but it was removed in https://github.com/sveltejs/svelte/pull/15348 Main: ``` { benchmark: 'sbench_create_signals', time: '401.26', gc_time: '29.60' } { benchmark: 'sbench_create_0to1', time: '8.17', gc_time: '0.00' } { benchmark: 'sbench_create_1to1', time: '29.06', gc_time: '0.59' } { benchmark: 'sbench_create_2to1', time: '27.08', gc_time: '0.52' } { benchmark: 'sbench_create_4to1', time: '25.51', gc_time: '0.61' } { benchmark: 'sbench_create_1000to1', time: '24.78', gc_time: '0.49' } { benchmark: 'sbench_create_1to2', time: '12.27', gc_time: '0.00' } { benchmark: 'sbench_create_1to4', time: '16.14', gc_time: '1.18' } { benchmark: 'sbench_create_1to8', time: '13.43', gc_time: '1.21' } { benchmark: 'sbench_create_1to1000', time: '4.89', gc_time: '0.00' } { benchmark: 'kairo_avoidable_owned', time: '415.52', gc_time: '77.30' } { benchmark: 'kairo_avoidable_unowned', time: '507.39', gc_time: '73.81' } { benchmark: 'kairo_broad_owned', time: '414.50', gc_time: '2.02' } { benchmark: 'kairo_broad_unowned', time: '413.50', gc_time: '2.16' } { benchmark: 'kairo_deep_owned', time: '181.87', gc_time: '1.93' } { benchmark: 'kairo_deep_unowned', time: '806.19', gc_time: '4.01' } { benchmark: 'kairo_diamond_owned', time: '340.28', gc_time: '34.45' } { benchmark: 'kairo_diamond_unowned', time: '434.26', gc_time: '34.75' } { benchmark: 'kairo_triangle_owned', time: '96.85', gc_time: '4.79' } { benchmark: 'kairo_triangle_unowned', time: '198.79', gc_time: '4.57' } { benchmark: 'kairo_mux_owned', time: '298.60', gc_time: '3.77' } { benchmark: 'kairo_mux_unowned', time: '450.16', gc_time: '3.95' } { benchmark: 'kairo_repeated_owned', time: '54.82', gc_time: '5.33' } { benchmark: 'kairo_repeated_unowned', time: '55.61', gc_time: '5.08' } { benchmark: 'kairo_unstable_owned', time: '88.33', gc_time: '5.46' } { benchmark: 'kairo_unstable_unowned', time: '103.47', gc_time: '4.81' } { benchmark: 'mol_bench_owned', time: '231.41', gc_time: '0.75' } { benchmark: 'mol_bench_unowned', time: '248.76', gc_time: '1.18' } Finished reactivity benchmarks. { suite_time: '5902.90', suite_gc_time: '304.32' } ``` This PR: ``` { benchmark: 'sbench_create_signals', time: '295.67', gc_time: '79.27' } { benchmark: 'sbench_create_0to1', time: '7.83', gc_time: '0.00' } { benchmark: 'sbench_create_1to1', time: '19.93', gc_time: '2.14' } { benchmark: 'sbench_create_2to1', time: '17.41', gc_time: '1.75' } { benchmark: 'sbench_create_4to1', time: '16.30', gc_time: '1.77' } { benchmark: 'sbench_create_1000to1', time: '14.97', gc_time: '2.05' } { benchmark: 'sbench_create_1to2', time: '9.86', gc_time: '0.60' } { benchmark: 'sbench_create_1to4', time: '19.46', gc_time: '6.63' } { benchmark: 'sbench_create_1to8', time: '15.18', gc_time: '3.71' } { benchmark: 'sbench_create_1to1000', time: '4.45', gc_time: '0.00' } { benchmark: 'kairo_avoidable_owned', time: '270.91', gc_time: '4.66' } { benchmark: 'kairo_avoidable_unowned', time: '362.95', gc_time: '5.16' } { benchmark: 'kairo_broad_owned', time: '408.44', gc_time: '0.00' } { benchmark: 'kairo_broad_unowned', time: '407.62', gc_time: '0.00' } { benchmark: 'kairo_deep_owned', time: '175.84', gc_time: '0.00' } { benchmark: 'kairo_deep_unowned', time: '805.32', gc_time: '0.00' } { benchmark: 'kairo_diamond_owned', time: '273.82', gc_time: '2.69' } { benchmark: 'kairo_diamond_unowned', time: '379.19', gc_time: '2.78' } { benchmark: 'kairo_triangle_owned', time: '87.13', gc_time: '0.41' } { benchmark: 'kairo_triangle_unowned', time: '186.83', gc_time: '0.43' } { benchmark: 'kairo_mux_owned', time: '291.71', gc_time: '1.28' } { benchmark: 'kairo_mux_unowned', time: '437.53', gc_time: '1.37' } { benchmark: 'kairo_repeated_owned', time: '44.12', gc_time: '0.21' } { benchmark: 'kairo_repeated_unowned', time: '45.87', gc_time: '0.21' } { benchmark: 'kairo_unstable_owned', time: '76.82', gc_time: '0.53' } { benchmark: 'kairo_unstable_unowned', time: '92.73', gc_time: '0.60' } { benchmark: 'mol_bench_owned', time: '230.92', gc_time: '0.24' } { benchmark: 'mol_bench_unowned', time: '248.28', gc_time: '0.39' } Finished reactivity benchmarks. { suite_time: '5247.09', suite_gc_time: '118.88' } ```