add code design

docs/code_design.md

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+# Code Design
+
+## Code Organization
+
+1. Every function except `main.ts` should be pure. Pass any stateful information in parameters.
+
+2. `misc.ts` should not depend on any other written code.
+
+3. Each storage code should not depend on `sync.ts`.
+
+## File and Folder Representation
+
+While writing sync codes, folders are always represented by a string ending with `/`.