Update lang-core & cli to move to spec generation

[AbhinRustagi]

Closes TH-2332

lang-core

1. Deprecates the existing generateSystemPrompt(spec: PromptSpec) function in favour of a new generateSystemPrompt(spec: { library: LibrarySpec, promptOptions?: SystemPromptOptions }) function
   • The new function is overloaded which checks for both types and return the response accordingly.
   • The old version will be dropped in next major release
2. Adds exports for the new types

CLI

1. Changes the behaviour of the existing generate command to now output both the system prompt file (for backwards compatibility) and the spec file.
   • The output is generated by the generate-worker.ts now which joins the prompt & spec by a SEPARATION_DELIMITER which is split during the write or stdout.

react-lang

1. Re-exports the new types from lang-core

docs

1. Updated CLI API Reference
2. Updated Guides that reference the old generatePrompt() or createResponsesInstructions() for @openuidev/thesys-server