Improve shinyapps io deploy

[avanlinden]

This PR updates the github actions workflow that deploys the dictionary app to shinyapps.io, which had gotten quite out of date.

• the new deployment workflow is based on this: https://github.com/r-lib/actions/blob/v2-branch/examples/shiny-deploy.yaml
• the updated workflow sets the rstudio public package repository as the default download environment for package installs, so it is much faster and installing from renv no longer bogs the job down
• I added an .rscignore file and removed the appFiles argument in rsconnect::deployApp() in order to prevent all the renv files from being added to the bundle sent via rsconnect