The TypeScript defs generation tool is called tsgen and is written in TypeScript. Build it by running npm run build-tsgen. This will compile the parser locally.
You can then run npm run tsgen to build the actual defs. They will replace the file located in the root types folder. Once the parser is built you only need use this command. Use it to re-generate the defs if you have modified the Phaser source code and wish to test your change worked.
There is also a test script available: npm run test-ts which will compile a test TypeScript project and output any compilation errors to output.txt.