Make `FS` smart about the year in generated license headers

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](https://github.com/grafana/grafana/blob/main/public/app/plugins/panel/barchart/models.cue#L1), 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.