Make FS smart about the year in generated license headers

[sdboyer]

As it is currently written, codejen.FS treats all bytes as undifferentiated. This is fine, for now. But when the year rolls over, lines like this, will be updated.

This isn't usually the way that license headers work - typically the year is left alone, once the file is created.

While i do want to avoid putting too much smarts into the codejen.FS, i'd be comfortable with a hack that sniffs specifically for a regex pattern like that in a file that it's writing to or verifying, and dynamically changing the bytes in its buffer to be the year that's in the file already on disk.