Make index map specification more precise

[takikawa]

Currently the structure of index maps are described in the spec like this:

It shares the version and file field from the regular source map, but gains a new sections field.

This makes it clear that version, file (optionally), and sections fields are present in an index map. What's not explicitly spelled out is if this excludes other fields such as mappings or sources in the top-level map or if that falls under "unrecognized properties", and if it must be an index map if a sections field exists.

In practice, implementations I've seen will check if the sections field is present and decide whether it's intended to be an index map based on that (and ignoring extra entries that are irrelevant to an index map). Maybe the spec can just codify this behavior?