libghostty: add utf-8 grapheme cell getter to C API

Add a render-state row-cells getter that encodes the current cell's
full grapheme cluster directly as UTF-8 into a caller-provided
GhosttyBuffer. The getter writes the base codepoint first, followed by
any extra grapheme codepoints, and follows the existing buffer-writer
convention where len is bytes written on success or required capacity
on GHOSTTY_OUT_OF_SPACE.

Previously C consumers could query grapheme codepoints, but bindings
that needed UTF-8 text had to reconstruct and encode the cluster
themselves. That duplicated terminal internals in downstream bindings
and made users pay for awkward cross-language struct handling. By
owning the UTF-8/grapheme behavior in libghostty, bindings can use one
stable C API and optionally wrap it with small binding-local helpers.