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.

As of Zig 0.14.0, `@splat` can be used for array types, which eliminates
a lot of redundant syntax and makes things generally cleaner.

I've explicitly avoided applying this change in the renderer files for
now since it would just create rebasing conflicts in my renderer rework
branch which I'll be PR-ing pretty soon.

Fixes #7337

AppKit encodes functional keys as PUA codepoints. We don't want to send
that down as valid text encoding for a key event because KKP uses that
in particular to change the encoding with associated text.

I think there may be a more specific solution to this by only doing this
within the KKP encoding part of KeyEncoder but that was filled with edge
cases and I didn't want to risk breaking anything else.