Add TypeScript migration documentation

See #1334 (comment)

Author: JamieMagee

docs/typescript-migration.md

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+# TypeScript Migration Strategy
+
+This document outlines our current TypeScript migration strategy for the ClearlyDefined service.
+We are taking a gradual, incremental approach to improve type safety while maintaining backward compatibility.
+
+## Current State
+
+### What We Have
+
+- **TypeScript Compiler Configuration**: We use TypeScript primarily for type checking JavaScript files via `tsconfig.json`
+- **Type Definition Files**: `.d.ts` files provide type information
+- **JSDoc Type Annotations**: Many JavaScript files include JSDoc comments for better IDE support and basic type checking
+- **TypeScript Checking**: The `tsc` command runs as part of our lint process to catch type errors
+
+### Current TypeScript Configuration
+
+See [`tsconfig.json`][tsconfig] for the current configuration.
+
+**Key aspects of our current setup:**
+
+- **Extends strictest TypeScript configuration**: Uses [`@tsconfig/strictest`][tsconfig-strictest] for maximum type safety, with [`@tsconfig/node18`][tsconfig-node18] for Node.js compatibility
+- **JavaScript type checking**: `allowJs: true` and `checkJs: true` enable TypeScript to analyze JavaScript files
+- **No compilation**: `noEmit: true` means we only do type checking, no compilation to JavaScript
+- **Selective strictness**: `strictNullChecks: false` and `exactOptionalPropertyTypes: false` are disabled as they're difficult to enforce in JavaScript
+- **Expanding coverage**: The `include` array covers many core library files and their type definitions
+
+## How to Add New Types
+
+### 1. Create Type Definition Files
+
+For JavaScript modules that need types, create corresponding `.d.ts` files
+
+### 2. Add JSDoc Type Annotations
+
+For existing JavaScript files, add JSDoc comments with type information
+
+### 3. Extend TypeScript Checking
+
+To include new files in TypeScript checking, add them to the `include` array in [`tsconfig.json`][tsconfig]:
+
+```json
+{
+  "include": [
+    // Existing files
+    // ...
+    "your/new/file.js" // Add new files here
+  ]
+}
+```
+
+## Future Migration Path
+
+### Near-term
+
+1. **Expand Type Coverage**: Add `.d.ts` files for more JavaScript modules
+1. **Enhanced JSDoc**: Improve existing JSDoc annotations across the codebase
+1. **Incremental TypeScript Checking**: Gradually add more files to the `include` list
+
+### Medium-term
+
+We have several options for deeper TypeScript integration:
+
+#### Option 1: Node.js Native TypeScript Support
+
+Node.js 22.6+ offers native TypeScript support with type stripping:
+
+**Advantages:**
+
+- No build step required
+- Lightweight type stripping only
+- Compatible with existing tooling
+
+**Requirements:**
+
+- [Node.js 22.6+ with `--experimental-strip-types` flag or Node.js 23.6+][node-typescript], or
+- [TypeScript 5.8+ with `erasableSyntaxOnly` enabled][ts-node-native]
+
+**Limitations:**
+
+- Only erasable TypeScript syntax (no enums, namespaces with runtime code, parameter properties)
+- Must use explicit `import type` syntax
+- File extensions required in imports (e.g., `import './file.ts'`)
+
+#### Option 2: Build-based TypeScript
+
+Traditional TypeScript compilation with a build step:
+
+**Advantages:**
+
+- Full TypeScript feature support
+- Better IDE integration
+- Stronger type checking
+
+**Requirements:**
+
+- Add build process to deployment pipeline
+- Update package.json scripts
+- Handle source maps and debugging
+
+## Getting Started
+
+To contribute type definitions:
+
+1. Identify a JavaScript module that would benefit from types
+1. Create a corresponding `.d.ts` file with appropriate interfaces and type definitions
+1. Use the types in JSDoc comments or import them in JavaScript files
+1. Add the file to the TypeScript `include` configuration in [`tsconfig.json`][tsconfig] if it's not already covered
+1. Test the types by running `npm run tsc`
+1. Update this document as we learn and evolve our approach
+
+[tsconfig]: ../tsconfig.json
+[tsconfig-strictest]: https://www.npmjs.com/package/@tsconfig/strictest
+[tsconfig-node18]: https://www.npmjs.com/package/@tsconfig/node18
+[node-typescript]: https://nodejs.org/api/cli.html#cli_node_experimental_strip_types
+[ts-node-native]: https://devblogs.microsoft.com/typescript/announcing-typescript-5-8-beta/#the---erasablesyntaxonly-option