Replace ragg PNG device with native SVG device for vignettes

[nanxstats]

This PR replaces the current graphical device for plots in vignettes (ragg PNG) with the R native SVG device.

This has a few benefits:

1. Makes plots look much sharper by using vector graphics.
2. Reduces the package tarball size from 1.5Mb to ~990KB.
3. Removes the soft dependency on ragg. ragg's hard dependency systemfonts is having trouble being built on r-oldrel-macos-arm64 on CRAN, which results in the following build error for gsDesign on that platform, for every vignette that used the ragg PNG device:

Error(s) in re-building vignettes:
--- re-building ‘ConditionalErrorSpending.Rmd’ using rmarkdown
  
Quitting from lines 54-58 [unnamed-chunk-2] (ConditionalErrorSpending.Rmd)
Error: processing vignette 'ConditionalErrorSpending.Rmd' failed with diagnostics:
there is no package called 'systemfonts'
--- failed re-building ‘ConditionalErrorSpending.Rmd’

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