Performance optimization

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Before:

cpu: 12th Gen Intel(R) Core(TM) i5-12450H
BenchmarkLexer/960.css.gz-12               14624             76186 ns/op               124.8 MiB/s         40640 B/op     1761 allocs/op
BenchmarkLexer/animate.css.gz-12            1374            887570 ns/op               101.7 MiB/s        612419 B/op    18777 allocs/op
BenchmarkLexer/blueprint.css.gz-12          7112            160003 ns/op               103.7 MiB/s         73504 B/op     3626 allocs/op
BenchmarkLexer/bootstrap-4.css.gz-12         536           2097289 ns/op                90.74 MiB/s      1271188 B/op    47858 allocs/op
BenchmarkLexer/bootstrap-5.css.gz-12         568           2194725 ns/op                89.04 MiB/s      1296619 B/op    48388 allocs/op
BenchmarkLexer/font-awesome-all.css.gz-12                   1764            678521 ns/op               102.8 MiB/s      374017 B/op      15512 allocs/op
BenchmarkLexer/foundation.css.gz-12                          711           1722624 ns/op                92.26 MiB/s      1024611 B/op      40907 allocs/op
BenchmarkLexer/inuit.css.gz-12                              4370            314816 ns/op               160.6 MiB/s        149043 B/op       5444 allocs/op
BenchmarkLexer/normalize.css.gz-12                         36096             31776 ns/op               184.0 MiB/s         12640 B/op        520 allocs/op
BenchmarkLexer/pure.css.gz-12                               5348            247496 ns/op               111.1 MiB/s        133200 B/op       5013 allocs/op
BenchmarkLexer/reset.css.gz-12                             87961             13562 ns/op                76.28 MiB/s         5504 B/op        304 allocs/op
BenchmarkLexer/tailwind.css.gz-12                             60          21809385 ns/op               103.9 MiB/s      16030122 B/op     491444 allocs/op

After:

cpu: 12th Gen Intel(R) Core(TM) i5-12450H
BenchmarkLexer/960.css.gz-12               27336             46168 ns/op               206.0 MiB/s            65 B/op          0 allocs/op
BenchmarkLexer/animate.css.gz-12            2169            515651 ns/op               176.1 MiB/s           520 B/op          1 allocs/op
BenchmarkLexer/blueprint.css.gz-12         13356             90538 ns/op               182.8 MiB/s           110 B/op          0 allocs/op
BenchmarkLexer/bootstrap-4.css.gz-12        1023           1081599 ns/op               176.2 MiB/s           997 B/op          2 allocs/op
BenchmarkLexer/bootstrap-5.css.gz-12        1117           1089872 ns/op               179.1 MiB/s          1034 B/op          2 allocs/op
BenchmarkLexer/font-awesome-all.css.gz-12                   3196            370056 ns/op               187.7 MiB/s           433 B/op          1 allocs/op
BenchmarkLexer/foundation.css.gz-12                         1219            940975 ns/op               170.0 MiB/s           776 B/op          1 allocs/op
BenchmarkLexer/inuit.css.gz-12                              6634            183621 ns/op               275.8 MiB/s           250 B/op          0 allocs/op
BenchmarkLexer/normalize.css.gz-12                         61032             20811 ns/op               281.1 MiB/s            48 B/op          0 allocs/op
BenchmarkLexer/pure.css.gz-12                               8744            129488 ns/op               212.0 MiB/s           145 B/op          0 allocs/op
BenchmarkLexer/reset.css.gz-12                            152024              7852 ns/op               132.4 MiB/s            27 B/op          0 allocs/op
BenchmarkLexer/tailwind.css.gz-12                            100          10623154 ns/op               213.7 MiB/s          2378 B/op          7 allocs/op