std.mem.Allocator `dupeZ` to `dupeSentinel`

[ANDRVV]

In the standard library, std.mem.Allocator has the dupeZ method, which provides no flexibility. Changing it to dupeSentinel could be a "Zig style" option that increases maintainability and code flexibility.

The dupeSentinel method should look like this:

pub fn dupeSentinel(allocator: Allocator, comptime T: type, m: []const u8, comptime s: T) Error![:s]T {
    const new_buf = try allocator.alloc(T, m.len + 1);
    @memcpy(new_buf[0..m.len], m);
    new_buf[m.len] = s;
    return new_buf[0..m.len :s];
}