Refactor read_tax()

[kelly-sovacool]

Change(s) made

• Moved most of read_tax() to a separate function parse_tax(), to separate file reading from parsing.
• Allow read_tax() to handle other delim formats (e.g. csv) with new sep parameter.

Checklist

(~Strikethrough~ any points that are not applicable.)

• ~[ ] Write unit tests for any new functionality.~
• [x] Update roxygen comments & vignettes if there are any API changes.
• [x] Update NEWS.md if this includes any user-facing changes.
• [x] The check workflow succeeds on your most recent commit.