Ship a static .gif file with the CRAN README

[MichaelChirico]

Much like we have *-still.gif for the vignettes on CRAN, we should also shrink the {lintr} package size by shipping a static version of this image:

https://github.com/r-lib/lintr/blob/main/man/figures/demo.gif

It currently occupies 16% of the package size on its own. I'd rather just point users to the GitHub page/website for an animated version.

That said, it requires more than a few seconds to make the change, since we currently use a fixed .md file; it will probably require switching to a README.Rmd.

lintr/README.md

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