wip migration document

Author: matcool

tutorials/migrate-v4.md

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+# Migrating from Geode v3.x to v4.0
+
+## Changes to `Result`
+* Result type has been rewritten from scratch, and it is now shared with many parts of the Geode codebase (Geode itself, TulipHook and matjson).
+* Many methods removed or renamed:
+    * `value()` -> `unwrap()`
+    * `expect(str, fmt args...)` -> Use `mapErr` along with `fmt::format`
+    * Snake case methods renamed to camel case
+* Universal `GEODE_UNWRAP(res)` and `GEODE_UNWRAP_INTO(value, res)` macros
+    * These macros were already previously available, but this is just a reminder that they can help a lot when dealing with results.
+    * If you only ever expect to build your code with **clang**, you can use the `GEODE_UNWRAP` macro like so, for convenience:
+    ```cpp
+    Result<int> getInt();
+
+    Result<int> foo(int extra) {
+        // This will not compile on MSVC
+        int value = GEODE_UNWRAP(getInt()) + extra;
+        return Ok(value);
+    }
+    ```
+
+## Changes to the Geode Settings API
+* Settings v2 has been removed, V3 suffix from classes are now aliased
+* Code that already used SettingsV3 should still work
+
+## Changes to `geode::Layout`
+* No longer in cocos2d namespace
+    * Can now be found in `Geode/ui/Layout.hpp`
+
+## Changes to `matjson`
+[Link to the full docs](https://github.com/geode-sdk/json)
+* Entire library rewritten to use `geode::Result`
+    * See result section for tips on how to use Geode's Result class
+* Methods are now camel case to fit with rest of Geode's codebase
+    * `is_string` -> `isString`
+    * `as_string` -> `asString`
+    * etc..
+* `matjson::Object` has been removed, now you can just iterate directly off a Value (same for arrays!)
+    ```cpp
+    matjson::Value someObject = ...;
+    for (auto& [key, value] : someObject) {
+        log::debug("{}: {}", key, value);
+    }
+
+    matjson::Value someArray = ...;
+    for (auto& value : someArray) {
+        log::debug("{}", value);
+    }
+    ```
+* Use `matjson::makeObject` for making objects inline now:
+    ```cpp
+    matjson::Value obj = matjson::makeObject({
+        { "key", 123 },
+        { "another-key": "another value!" }
+    });
+    ```
+* Accessing missing properties with `Value::operator[]` will now return a null value instead of throwing an exception
+* Serialization methods have been changed:
+    * `T from_json(matjson::Value const&)` -> `Result<T> fromJson(matjson::Value const&)`
+    * `matjson::Value to_json(T const& value)` -> `matjson::Value toJson(T const& value)`
+    * `is_json` is no longer used, instead just return an error in `fromJson`
+* `bool Value::is<T>()` removed
+* `#include <matjson/stl_serialize.hpp>` -> `#include <matjson/std.hpp>`
+* Added new experimental `<matjson/reflect.hpp>` header, uses [qlibs/reflect](https://github.com/qlibs/reflect) to (de)serialize aggregate structs
+    * Behavior might change in the future, so use with caution