input: use std.Io.Writer for key encoder, new API, expose via libghostty

This modernizes `KeyEncoder` to a new `std.Io.Writer`-based API.
Additionally, instead of a single struct, it is now an `encode` function
that takes a series of more focused options. This is more idiomatic Zig
while also making it easier to expose via libghostty-vt.

libghostty-vt also gains access to key encoding APIs.

src/input/keyboard.zig

@@ -1,5 +1,5 @@
 const std = @import("std");
-const OptionAsAlt = @import("../config.zig").OptionAsAlt;
+const OptionAsAlt = @import("config.zig").OptionAsAlt;
 
 /// Keyboard layouts.
 ///