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/config/Config.zig

@@ -2861,7 +2861,7 @@ keybind: Keybinds = .{},
 ///
 /// The values `left` or `right` enable this for the left or right *Option*
 /// key, respectively.
-@"macos-option-as-alt": ?OptionAsAlt = null,
+@"macos-option-as-alt": ?inputpkg.OptionAsAlt = null,
 
 /// Whether to enable the macOS window shadow. The default value is true.
 /// With some window managers and window transparency settings, you may
@@ -4821,14 +4821,6 @@ pub const NonNativeFullscreen = enum(c_int) {
     @"padded-notch",
 };
 
-/// Valid values for macos-option-as-alt.
-pub const OptionAsAlt = enum {
-    false,
-    true,
-    left,
-    right,
-};
-
 pub const WindowPaddingColor = enum {
     background,
     extend,