fix: decode the BS byte as backspace

[meszmate]

Fixes #146.

Backspace did nothing in the text editor on xterm. xterm sends 0x08 for Backspace unless backarrowKey is turned off, but the parser only treated DEL as backspace and folded 0x08 into the Ctrl+letter range, so it arrived as ctrl+h. TextArea and TextInput return early on any ctrl key they don't recognise, so the event was dropped. Same reason Ctrl+D and Ctrl+Q kept working while Backspace and Ctrl+H did not.

0x08 now decodes to a plain backspace, and Kitty keycode 8 is accepted alongside 127.

The tradeoff: a legacy encoding cannot tell Backspace and Ctrl+H apart, they are the same byte. So Ctrl+H is only bindable under the Kitty protocol, which reports it separately as CSI 104;5u. There is a test pinning that. crossterm and vaxis make the same call, and nothing in this repo binds Ctrl+H.

Verified by driving the built binaries through a pty with TERM=xterm-256color, sending the exact bytes xterm sends. Showcase, tab 5, typing ABC then two backspaces:

before:  1 ABCconst std = @import("std");
after:   1 Aconst std = @import("std");

zig build and zig build test pass.