write the project's version to a version.ts file, and only if it exists

At the moment lume release* command try to write the project version to the last line of src/index.ts, and it will fail if src/index.ts does not have this line.

This should be optional, and should be opted-into by making a src/version.ts file whose whole contents will be replaced. The initial file can be empty,to be populated by lime cli.