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.

The [fixterms](http://www.leonerd.org.uk/hacks/fixterms/) "Really
Special Keypresses" section suggests using CSI 1 ; Ps R for F3, but this
is also a valid cursor position report. The intention was to make
back-compatible changes, so this is fairly considered a specification
bug.

This changes F3 in legacy mode to send CSI 13 ; Ps ~ instead, this is a
variant listed in fixterms, is what kitty protocol uses, and lacks the
problematic overlap with cursor positions.

The KeyEncoder.zig unit test has been changed accordingly, and all tests
pass on my machine.

The [fixterms](http://www.leonerd.org.uk/hacks/fixterms/) "Really
Special Keypresses" section suggests using CSI 1 ; Ps R for F3, but this
is also a valid cursor position report.  The intention was to make back-
compatible changes, so this is fairly considered a specification bug.

This changes F3 in legacy mode to send CSI 13 ; Ps ~ instead, this is a
variant listed in fixterms, is what kitty protocol uses, and lacks the
problematic overlap with cursor positions.

The KeyEncoder.zig unit test has been changed accordingly, and all tests
pass on my machine.

The comment in `function_keys.zig` was missing the `>` character for the
sequence.  I've confirmed that this was just the comment, Ghostty treats
the original as an SGR sequence, which it is.  Conversely, it does treat
`\x1b[>4:2m` as activating modifyOtherKeys.

Fixes #1099