[FEATURE] Support Date Times Using reference_event

[rsh52]

Feature Request Description

There is currently a reference_event argument for ggswim() which has not been incorporated into the business logic.

The idea here is that if time is not an integer/numeric value, i.e. Date/Date-time, that record swim lanes are centered automatically around a reference event from the event stream.

Additional Context

Example:

df <- tibble::tribble(
  ~subject_id, ~time, ~event,
  1, "2020-01-01", "Infusion",
  1, "2020-02-01", "CRS Grade 1",
  1, "2020-03-01", "CRS Grade 2",
  2, "2020-05-01", "Infusion",
  2, "2020-06-01", "No CRS",
  3, "2020-09-01", "Infusion",
  3, "2020-10-01", "CRS Grade 3",
...
)

ggswim(...
reference_event = "infusion",
...
)

Checklist

• The issue is atomic
• The issue description is documented
• The issue title describes the problem succinctly
• Developers are assigned to the issue
• Labels are assigned to the issue