addUserInputOption

pub fn addUserInputOption(b: *Build, name: []const u8, value_raw: []const u8) error{OutOfMemory}!bool {
    const graph = b.graph;
    const arena = graph.arena;
    const value = graph.dupeString(value_raw);
    const gop = try b.user_input_options.getOrPut(arena, name);

    if (!gop.found_existing) {
        gop.key_ptr.* = graph.dupeString(name);
        gop.value_ptr.* = .{ .scalar = value };
        return false;
    }

    // Option already exists.
    switch (gop.value_ptr.*) {
        .scalar => |s| {
            // Turn it into a list.
            var list: std.ArrayList([]const u8) = .empty;
            (try list.addManyAsArray(arena, 2)).* = .{ s, value };
            gop.value_ptr.* = .{ .list = list };
        },
        .list => |*list| try list.append(arena, value),
        .flag => {
            log.err("option -D{s}={s} conflicts with flag -D{s}", .{ name, value, name });
            return true;
        },
        .map => |*map| {
            _ = map;
            unreachable; // TODO implement maps as command line arguments
        },
        .lazy_path => unreachable,
        .lazy_path_list => unreachable,
    }
    return false;
}