Create ROADMAP.md

ROADMAP.md

+# Roadmap
+The order of individual tasks is a guide only and is subject to change depending on the situation.
+Also, the later tasks are more indefinite and are subject to change as development progresses.
+
+## (1) Improve maintainability \<current stage\>
+- Make the number of type errors zero (backend)
+  - Probably need to switch some libraries to others that make it difficult to reduce type errors
+    - e.g. koa to fastify https://github.com/misskey-dev/misskey/issues/7537
+- Improve CI
+  - Fix tests
+    - mocha, jest, etc. do not support the combination of `TypeScript + ESM + Path alias`, and the tests currently do not work.
+  - Fix random test failures - https://github.com/misskey-dev/misskey/issues/7985 and https://github.com/misskey-dev/misskey/issues/7986
+  - Add more tests
+    - May need to implement a mechanism that allows for DI
+- Improve documentation
+
+## (2) Improve functionality
+- OAuth2 support https://github.com/misskey-dev/misskey/issues/8262
+
+## (3) Improve scalability
+- Rewriting in Rust?