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

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,

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

Start with the epi_archive API and the six operator behaviors named in the issue: $, $<-, [, [<-, [[, and [[<-. Clarify the intended handling of partial matches, typos, and any version-metadata revalidation; done should prevent accidental partial matches while documenting the chosen behavior.

Written by the indexing model from the issue text.