fix(gatsby-remark-images): allow default max-width to be overwritten with wrapperStyle

[birjj]

Description

Currently gatsby-remark-images enforces a max-width on each image equal to the presentationWidth returned by gatsby-plugin-sharp. Any user-specified max-width in the wrapperStyle is simply ignored.
Use case for custom max-width is described in #15578.

This PR adds support for specifying a max-width in the wrapperStyle which will be used instead of the presentationWidth.

Previously submitted half a year ago in #15952, but abandoned by me. This PR uses the cleaner solution suggested by @pieh.

Related Issues

Fixes #15578.

[pieh]

Looks great, thanks!

[gatsbot]

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