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/function_keys.zig

@@ -293,6 +293,11 @@ fn pcStyle(comptime fmt: []const u8) []Entry {
 
 test "keys" {
     const testing = std.testing;
+    switch (@import("terminal_options").artifact) {
+        .ghostty => {},
+        // Don't want to bring in termio into libghostty-vt
+        .lib => return error.SkipZigTest,
+    }
 
     // Force resolution for comptime evaluation.
     _ = keys;