CLI - `spacetime generate` should not overwrite non-SpacetimeDB files

[bfops]

Background

As @jdetter noted in https://github.com/clockworklabs/SpacetimeDB/pull/1774/files#r1781948491, spacetime generate might happily overwrite files that were user-generated.

Definition of Done

• [ ] spacetime generate should only (over)write a file if it either doesn't exist, or if it starts with the "this file is autogenerated by SpacetimeDB" header

Priority

@bfops honestly doesn't think this should be a P1, but others could reasonable disagree. Marking this as a P1 so we have the discussion.