One that gets me 90% there would take me few hours, one that gets me 99% there few months, which is why eventually people would rather pull a dependency.
LLMs are pretty good at greenfield projects and especially if they are tasked with writing something with a lot of examples in the training data. This approach can be used to solve the problem of supply-chain attacks with the downside being that the code might not be as well written and feature complete as a third-party package.
Not for the parser, only for the demo server! And I guess the dev dependencies as well, but with a much smaller surface area. But yeah, I don't think a TypeScript compiler is within the scope of an LLM.