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/config.zig

@@ -0,0 +1,8 @@
+/// Determines the macOS option key behavior. See the config
+/// `macos-option-as-alt` for a lot more details.
+pub const OptionAsAlt = enum(c_int) {
+    false,
+    true,
+    left,
+    right,
+};

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;

src/input/key.zig

@@ -2,7 +2,7 @@ const std = @import("std");
 const builtin = @import("builtin");
 const Allocator = std.mem.Allocator;
 const cimgui = @import("cimgui");
-const config = @import("../config.zig");
+const OptionAsAlt = @import("config.zig").OptionAsAlt;
 
 /// A generic key input event. This is the information that is necessary
 /// regardless of apprt in order to generate the proper terminal
@@ -146,7 +146,7 @@ pub const Mods = packed struct(Mods.Backing) {
     /// Return the mods to use for key translation. This handles settings
     /// like macos-option-as-alt. The translation mods should be used for
     /// translation but never sent back in for the key callback.
-    pub fn translation(self: Mods, option_as_alt: config.OptionAsAlt) Mods {
+    pub fn translation(self: Mods, option_as_alt: OptionAsAlt) Mods {
         var result = self;
 
         // macos-option-as-alt for darwin

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.
 ///