feat(unicode): accept unicode characters through the app's input handler to be handled correctly; key introduce isUnicode method

With the `isUnicode` method the read unicode characters from the user
can be checked against displayable text and rendered on the screen accordingly.

examples/continuous.zig

@@ -87,7 +87,7 @@ const InputField = struct {
         const this: *@This() = @ptrCast(@alignCast(ctx));
         switch (event) {
             .key => |key| {
-                if (key.isAscii()) try this.input.append(this.allocator, key.cp);
+                if (key.isUnicode()) try this.input.append(this.allocator, key.cp);
 
                 if (key.eql(.{ .cp = zterm.input.Enter }) or key.eql(.{ .cp = zterm.input.KpEnter }))
                     this.queue.push(.{ .accept = try this.input.toOwnedSlice(this.allocator) });