Implement Sixel encoder/decoder support

[aymanbagabas]

PR originally implemented by @CannibalVox

Ref: https://github.com/charmbracelet/x/pull/352

Most of this PR changes are just updates on the implementation per requests

Supersedes: #380

[raphamorim]

@aymanbagabas tested locally and this branch had a regression in performance compared to #380 and #352

ansi/sixel feat-sixel-support-pp - go test -bench=. -benchmem -count 10
goos: darwin
goarch: arm64
pkg: github.com/charmbracelet/x/ansi/sixel
cpu: Apple M4 Pro
BenchmarkEncodingGoSixel-14    	       4	 295443552 ns/op	11894708 B/op	 1035163 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 293885792 ns/op	11896036 B/op	 1035164 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 304176646 ns/op	11894780 B/op	 1035164 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 301280604 ns/op	11894680 B/op	 1035163 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 299805969 ns/op	11894680 B/op	 1035163 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 301399333 ns/op	11894688 B/op	 1035163 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 300609573 ns/op	11894736 B/op	 1035163 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 300067042 ns/op	11894688 B/op	 1035163 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 300933344 ns/op	11894696 B/op	 1035163 allocs/op
BenchmarkEncodingGoSixel-14    	       4	 302264438 ns/op	11894728 B/op	 1035163 allocs/op
BenchmarkEncodingXSixel-14     	       4	 297760062 ns/op	43927074 B/op	  636054 allocs/op
BenchmarkEncodingXSixel-14     	       4	 298991354 ns/op	43927074 B/op	  636054 allocs/op
BenchmarkEncodingXSixel-14     	       4	 298944302 ns/op	43927074 B/op	  636054 allocs/op
BenchmarkEncodingXSixel-14     	       4	 301061729 ns/op	43927102 B/op	  636054 allocs/op
BenchmarkEncodingXSixel-14     	       4	 299697396 ns/op	43927106 B/op	  636054 allocs/op
BenchmarkEncodingXSixel-14     	       4	 302270990 ns/op	43927070 B/op	  636054 allocs/op
BenchmarkEncodingXSixel-14     	       4	 299696969 ns/op	43926950 B/op	  636052 allocs/op
BenchmarkEncodingXSixel-14     	       4	 302977177 ns/op	43926986 B/op	  636053 allocs/op
BenchmarkEncodingXSixel-14     	       4	 301184490 ns/op	43927074 B/op	  636054 allocs/op
BenchmarkEncodingXSixel-14     	       4	 300504781 ns/op	43927050 B/op	  636054 allocs/op
PASS
ok  	github.com/charmbracelet/x/ansi/sixel	48.628s