feat(gatsby-remark-images): handle image references

[artbycrunk]

Description

Hey guys this is a fix for gatsby-remark-images to pickup and process imageReference nodes correctly.
enabling the following syntax

[header]: header.jpg "Logo Title Text 2"

![alt text][header]

Related Issues

Fixes #11085

[DSchau]

This is looking great. Would you be able to add any tests and/or snapshots to validate this functionality?

[artbycrunk]

@DSchau have added in some test and snapshots to validate the following

it transforms image references in markdown (snapshot)
it leaves orphan image references alone
it transforms multiple image references in markdown
it transforms multiple image links and image references in markdown

[artbycrunk]

is this ok to merge?

[pieh]

I've added passing alt field from referencing image (it was discarded previously)
@artbycrunk please take a looksy in case you can think if doing it in nicer way (I didn't want to mutate reference node so created this overWrites object)

[pieh]

Let's merge this in. We can potentially iterate on implementation in future, there's no reason not to merge and publish.

Thanks @artbycrunk!

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