Fix: Remove error union from Program.init*

[IridescentRose]

While writing my TUI application I noticed my refactored main had some code that looked like it shouldn't compile:

pub fn main(init: std.process.Init) u8 {
    // ...
    var program = try zz.Program(tui.Model).initWithOptions(
        arena.allocator(),
        init.io,
        init.environ_map,
        .{
            .kitty_keyboard = true,
            .bracketed_paste = true,
            .title = "Hanabi",
            .alt_screen = true,
        },
    );
    // ...
    return 0;
}

try without !u8 as the return type should fail in most contexts; The reason it doesn't here is because the error set is {} -- there are no possible errors from the code path.

Checking init, nothing currently throws an error, it's all infallible struct initialization. Reflecting this appropriately in the type signature keeps the API honest to the real behavior, as it currently is impossible to throw an error. If at some point this changes, error-unions can be reintroduced then.