Proposal to rename Null->Nullable, Nullable->Nullish

该提案涉及 Null 和 Nullable 模块、它们生成的 JavaScript 行为，以及可能的 ReScript 配置功能标志。由于没有指定文件、测试或入口点，请先定位这些模块，并审查针对 v12–v14 提出的迁移计划。在实现之前就命名和迁移策略达成一致后，才算完成。

I'm perfectly fine with naming we had all this time, but in a new era AI writes most of the code, and I think having a naming parity with JavaScript is more important. For example, this proposal was inspired by the Claude Code session where it was doing bindings for napi-rs and it used Nullable.t type thinking that its value is null, like in TS. I had to correct it myself, saying that it should use a more correct Null.t type.

Additionally, Null.t by itself is already confusing, making me subconsciously think that it's just a null value type.

Proposed migration plan

Since this is a very big breaking change and updating both modules at the same time will be an issue because it might silently change generated JavaScript code, I think we must approach it in multiple major releases. And because of this, start earlier. Here is the proposed plan.

1. In next v12 minor we deprecate Nullable module in favor of the new Nullish module
2. In v13 we we remove Nullable in favor of Nullish
3. In v14 we deprecate Null module in favor of Nullable - I think there should be a whole major version without Nullable module to prevent issues during migration

To accelerate the change, we can add a feature flag in ReScript config which would allow us to explicitly force the migration: Null->Nullable, Nullable->Nullish

Let me know what you think.