Adjust epi_slide_opt output naming; adjust epi_slide* autogrouping; fix tidyselect issue

[brookslogan]

Checklist

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Change explanations for reviewer

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Does half of Add .suffix (and .prefix?) arguments to slides #542.
• Resolves Make epi_slide_opt work with !!varname, maybe !!varnames, maybe !!!varnames #562

[brookslogan]

• resolve styler vs linter opinions on line length
• check whether naming overrides forwarded through _mean and _sum
• [-] add .prefix and .suffix naming features to base epi[x]_slide(?) --- but they are not the same feature... .prefix and .suffix for epi[x]_slide would be on the output columns, not on the input columns, and would not need to be glue syntax... might be confusing.
• Update NEWS.md
• [-] vignette updates?
  • Just did minimal changes to keep them running for now. Really should present epi_slide_{sum,mean} first, then the general version.
• auto-grouping&ungrouping by epikey, and ensure epi_slide also auto-ungroups if auto-grouping?
• Update epipredict to not break