libghostty: Remove all libc++ and libc++ ABI dependencies

[mitchellh]

This updates simdutf to my fork which has a SIMDUTF_NO_LIBCXX option that removes all libc++ and libc++ ABI dependencies. The plan is to open an upstream PR with this, but I want to verify it here first.

From there, the hand-written simd code we have has been updated to also no longer use any libc++ features. Part of this required removing utfcpp since it depended on libc++ (<iterator>).

libghostty-vt now only depends on libc.

Benchmark Results

Corpus	Current HEAD median	main median	Delta vs main	Notes
valid-mixed-1g-seed1.bin	9.245s	9.111s	1.5% slower	Near tie; main remains slightly faster on fully valid input
malformed-mixed-1g-seed1-rate0.005.bin	9.251s	12.705s	37.3% faster	Large improvement on malformed UTF-8 input

Approximate throughput from the medians:

• Valid corpus: current HEAD 110.8 MiB/s, main 112.4 MiB/s
• Malformed corpus: current HEAD 110.7 MiB/s, main 80.6 MiB/s

[mitchellh]

I actually found a way to do replacement character decoding more in concert with simdutf. Our invalid utf8 path is probably a lot faster now, but will verify with benchmarks soon!