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.

This is stomping towards minimizing our build.zig dependencies so that
it can be cached more often. Right now, touching almost any file in the
project forces the build.zig to rebuild which is destroying my
productivity.

Adds support for background images via the `background-image` config.

Resolves #3645, supersedes PRs #4226 and #5233.

See docs of added config keys for usage details.