introduce pointer stability safety locks to MultiArrayList

[andrewrk]

Extracted from #17719.

This issue is to introduce a similar API to MultiArrayList.

Related:

• introduce pointer stability safety locks to array lists #19326
• add stack traces to pointer stability safety locks #19328

[sno2]

Should stability locks fail in cases where pointers could be invalidated, but they were not? e.g.

    const Data = struct { a: u32, b: u8 };
    var list = MultiArrayList(Data){};
    defer list.deinit(testing.allocator);

    try list.ensureTotalCapacity(testing.allocator, 1);
    list.lockPointers();
    try list.append(testing.allocator, .{ .a = 0, .b = 3 }); // should this fail?

Nevermind, seems like precedence is to fail- https://github.com/ziglang/zig/pull/17719/files#diff-8d3864cfd9fd2f29dd2b0458387c1036858fa8ac78d64517736675fbe3eaf33cR1045-R1049

[nektro]

yes because in that case where u already did an ensure capacity call, you should use appendAssumeCapacity instead of append.