perf(css): Introduce CSS validation cache

[PHPGangsta]

This change delivers a significant performance improvement. I benchmarked 30 HTML fixtures ranging from 600 bytes to 1.5 MB, and the overall runtime dropped by ~30% on average.

[PHPGangsta]

Here my performance comparison (time columns in seconds):

Filename	Size	v4.19.0	v4.19.0	% improvement
file_1.html	16.79 KB	0.012074	0.009871	18.25%
file_2.html	174.69 KB	0.052382	0.044274	15.48%
file_3.html	1.55 MB	0.030671	0.024884	18.87%
file_4.html	223.8 KB	0.110211	0.096668	12.29%
file_5.html	220.73 KB	0.214735	0.205500	4.30%
file_6.html	1.78 KB	0.003513	0.003002	14.55%
file_7.html	677 B	0.002160	0.001467	32.08%
file_8.html	807.24 KB	0.517928	0.198742	61.63%
file_9.html	63.85 KB	0.024996	0.023156	7.36%
file_10.html	69.78 KB	0.040163	0.033159	17.44%
file_11.html	16.19 KB	0.008988	0.006415	28.63%
file_12.html	33.53 KB	0.027129	0.026988	0.52%
file_13.html	1.35 KB	0.001827	0.001761	3.61%
file_14.html	254.43 KB	0.082556	0.074510	9.75%
file_15.html	1.31 MB	0.556819	0.508497	8.68%
file_16.html	2.35 KB	0.002718	0.001942	28.55%
file_17.html	1.83 KB	0.002005	0.001303	35.02%
file_18.html	1.92 KB	0.002295	0.002173	5.32%
file_19.html	4.21 KB	0.016169	0.011155	31.01%
file_20.html	2.03 KB	0.001159	0.001132	2.32%
file_21.html	1.85 KB	0.001468	0.001477	-0.62%
file_22.html	2.14 KB	0.096445	0.090994	5.65%
file_23.html	8.73 MB	3.806175	2.608822	31.46%
file_24.html	6.91 MB	3.560399	2.392689	32.80%
file_25.html	27.22 MB	15.081169	9.677023	35.83%
file_26.html	15.43 KB	0.012349	0.012214	1.09%
file_27.html	3.35 KB	0.001129	0.000951	15.76%
file_28.html	38.21 KB	0.029521	0.031496	-6.69%
file_29.html	685.83 KB	0.417299	0.184710	55.74%
file_30.html	885.84 KB	0.598710	0.223569	62.66%

Summary stats

	v4.19.0	v4.19.0	% improvement
Memory Usage	727 MB	733.69 MB
Memory Real Usage	797.23 MB	799.67 MB
Seconds	25.361588	16.553331	34.73%

PHP Version: 8.1.34