Add normality check for "random effects" for afex_aov

[mattansb]

For within/mixed designs, similiar to check_normality(<merMod>).

• for check_normality(<afex_aov>):
  • Detect the random deviations
  • extract them (per-term)
  • run test
• add plotting option in {see}.
• Update afex (vignette("assumptions_of_ANOVAs", package = "afex"))

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See old attempt here: singmann/afex@c97cab8
And the proj function.