Allow write/read/use of source mappings to/from needs.json

[PhilipPartsch]

Currently the lineno is been excluded from the needs.json generation. This is bad, as following tools cannot fetch the location of the needs.
So we want to add them to the needs.json export.

[chrisjsewell]

Thanks @PhilipPartsch, firstly I would suggest changing the title to something like "Allow write/read/use of source mappings to/from needs.json", as I imagine lineno will probably not be useful on its own?

Secondly, I would suggest this should be under a configuration flag, since this is possibly not the general use case for external needs, and also it is maybe somewhat in opposition to #1082 (where we are trying to reduce the size of needs.json)

[PhilipPartsch]

I would made lineno and content_offset mandatory, as we need these information for a correct warning / error generation if content of needs are creating an issue by rendering.