lib-vt: wasm convenience functions and a simple example (#9309)

This adds a set of Wasm convenience functions to ease memory management.
These are all prefixed with `ghostty_wasm` and are documented as part of
the standard Doxygen docs.

I also added a very simple single-page HTML example that demonstrates
how to use the Wasm module for key encoding.

This also adds a bunch of safety checks to the C API to verify that
valid values are actually passed to the function. This is an easy to hit
bug.

**AI disclosure:** The example is AI-written with Amp. I read through
all the code and understand it but I can't claim there isn't a better
way, I'm far from a JS expert. It is simple and works currently though.
Happy to see improvements if anyone wants to contribute.

src/input/key_encode.zig

@@ -77,7 +77,7 @@ pub fn encode(
     event: key.KeyEvent,
     opts: Options,
 ) std.Io.Writer.Error!void {
-    //std.log.warn("KEYENCODER event={} opts={}", .{ event, opts });
+    std.log.warn("KEYENCODER event={} opts={}", .{ event, opts });
     return if (opts.kitty_flags.int() != 0) try kitty(
         writer,
         event,