fix(ext-gen): skip `README.md` generation in extension-init command if it exists (#2283)
Currently, the `extension-init` command automatically generates a
boilerplate `README.md`. While helpful for initial setups, this behavior
becomes destructive during the iterative development phase. If a
developer has customized their README and later runs `extension-init` to
update function signatures or add new functions, their custom
documentation is overwritten without warning.