input: use std.Io.Writer for key encoder, new API, expose via libghostty (#9030)

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 Zig module also gains access to key encoding APIs. The C
APIs will follow another time.

Converting the KeyEncoder tests was done using AI:
https://ampcode.com/threads/T-9731bbdc-e0a9-41ad-9404-2b781a66ee39
Reviewed and understood.

0 files changed, 0 insertions, 0 deletions