docs: extract focus encoding example into standalone project

Extract the inline code example from focus.h into a standalone
buildable example at example/c-vt-encode-focus. The header now
uses a Doxygen @snippet tag to include the code from the example
source file, so the documentation stays in sync with code that
is verified to compile and run.

example/c-vt-encode-focus/README.md

@@ -0,0 +1,17 @@
+# Example: `ghostty-vt` Encode Focus
+
+This contains a simple example of how to use the `ghostty-vt` focus
+encoding API to encode focus gained/lost events into escape sequences.
+
+This uses a `build.zig` and `Zig` to build the C program so that we
+can reuse a lot of our build logic and depend directly on our source
+tree, but Ghostty emits a standard C library that can be used with any
+C tooling.
+
+## Usage
+
+Run the program:
+
+```shell-session
+zig build run
+```