feat(terminal): add support for copying with OSC 52 in embedded terminal (#29117)

When libvterm receives the OSC 52 escape sequence it ignores it because
Nvim does not set any selection callbacks. Install selection callbacks
that forward to the clipboard provider, so that setting the clipboard
with OSC 52 in the embedded terminal writes to the system clipboard
using the configured clipboard provider.

src/nvim/base64.c

@@ -142,6 +142,7 @@ char *base64_encode(const char *src, size_t src_len)
 /// @param [out] out_lenp Returns the length of the decoded string
 /// @return Decoded string
 char *base64_decode(const char *src, size_t src_len, size_t *out_lenp)
+  FUNC_ATTR_NONNULL_ALL
 {
   assert(src != NULL);
   assert(out_lenp != NULL);