Convert function-specific percent function themes to pkgwide

[ddsjoberg]

We don't need a theme element for each function, when a user just wants to set the way percentages are rounded/formatted throughout the package.

[ddsjoberg]

I went to go make this change, but there is only one place that this theme element is called. perhaps it would be easier to just document the element a bit further so it's clear this also includes tbl_svysummary(), tbl_cross(), tblwide_summary(), etc.