feat: Use "cosmo" bootswatch only if neither preset nor bootswatch are set

[gadenbuie]

This is a small change that will be transparent to most users that avoids errors if setting the theme: preset key. Currently, because bslib errors if preset and bootswatch are both set, and because bootswatch: cosmo is set by default by flexdashboard, users would need to write

output:
  html_document:
    theme:
      version: 5
      preset: shiny
      bootswatch: ~

to avoid the error from bslib. After this change, the bootswatch: ~ line isn't needed.