This adds a `clipboard_read` effect to the stream terminal handler and a
matching `GHOSTTY_TERMINAL_OPT_CLIPBOARD_READ` callback to the
libghostty-vt C API so that embedders can answer OSC 52 read requests
(the `?` payload).
This is a _blocking_ effect: if the embedder needs to ask the user for
permission, the entire VT processing pipeline is _blocked_ during the
callback. This is a purposeful simplification choice compared to how
Ghostty GUI works with async requests. I think its reasonable, it
eliminates a TON of complexity.
If the effect isn't set, then any clipboard reads are denied.
This can be expanded easily to support Kitty clipboard protocol later.
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