Make precise what is accepted as an index in, e.g., ignoreList

[takikawa]

A few different fields are encoded as indices into an array, and it may be worth being precise about what are acceptable values for these indices. For source/name indices, since the mappings string already constrains the representation, there's not much more to clarify other than possibly to note that out-of-bounds or negative indices are invalid.

For entries in ignoreList, the spec says it's an index into the source array:

ignoreList an optional list of indices of files that should be considered third party code, such as framework code or bundler-generated code. <...> It refers to the [sources](https://tc39.es/source-map/#sources) array and lists the indices of all the known third-party sources in the source map.

To make this more precise, we could add a parenthetical note like

ignoreList an optional list of indices (non-negative integers) of files <...>

to rule out indices like "1" or 1.5 etc.