Add name-validating, non-partial-matching `$`, `$<-`, `[`, `[<-`, `[[`, `[[<-` for `epi_archive`

[brookslogan]

We might also consider some partial or full revalidation of the result (e.g. checking version metadata bounds). Main thing is to prevent accidental partial matches and catch typos. For instance,

# Actual member variable
archive_cases_dv_subset$versions_end
# Partial matches to the same
archive_cases_dv_subset$version