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.

Compromise solution to #7356

XKB is naughty. It's really really naughty. I don't understand why we
didn't just kill XKB with hammers during the Wayland migration and change
it for something much better. I don't understand why we're content with
what amounts to an OS-level software key remapper that completely jumbles
information about original physical key codes in order to fake keyboard
layouts, and not just let users who really want to remap keys use some
sort of evdev or udev-based mapper program.

In a sane system like macOS, the "c" key is always the "c" key, but it's
understood to produce the Unicode character "ц" when using a Russian
layout. XKB defies sanity, and just pretends that your "c" key is
actually a "ц" key instead, and so when you ask for the keybind "Ctrl+C"
it just shrugs in apathy (#7309). And so, we took matters into our own
hands and interpreted hardware keycodes ourselves.

But then, a *lot* of people have the ingrained muscle memory of swapping
Escape with Caps Lock so that it is easier to hit. We respect that.
In a sane system, they would use a remapper that actually makes the
system think you've hit the Escape key when in reality you've hit the
Caps Lock key, so in all intents and purposes to the OS and any app
developer, these two just have their wires swapped. But not on Linux.
Somehow this and the aforementioned case should be treated by the same
key transform algorithm, which is completely diabolical.

As a result, we have to settle for a compromise that truly satisfies
neither party — by allowing XKB remaps for keys that don't really change
depending on the layout.

The Linux input stack besets all hopes and aspirations.

The origin of these keys are old sun keyboards.
They are getting picked up by the custom (progammable) keyboard scene
(see https://github.com/manna-harbour/miryoku for a popular layout).
Support in ghosty is quite handy because it allows to bind copy/paste in
a way that doesn't overlap with ctrl-c/ctrl-v, which can have special
bindings in some terminal applications.

Fixes #7310