Previously we had decided to always throw on these strings. Based on
feedback from the IETF SEDATE working group, we are going to allow them.
This means changes to what kinds of strings are allowed in several
contexts. See #1695 for more information.

The following diagram may be helpful when reading this commit.

"datetime" = e.g. "2021-08-19T17:30"
"offset" = e.g. "-07:00"
"bracket" = e.g. "[America/Vancouver]", "[-07:00]" or "[UTC]"

TimeZone.from:
  - datetime + Z -> UTC
  - datetime + offset -> offset time zone
  - datetime + bracket -> IANA time zone
  - datetime + Z + bracket -> "
  - datetime + offset + bracket -> "
  - throws on a bare datetime string

ZonedDateTime.from:
  - datetime + bracket -> preserve wall time in the IANA time zone
  - datetime + Z + bracket -> preserve exact time in the IANA time zone
  - datetime + offset + bracket -> consult offset option if ambiguous
  - throws on a string with no bracket

Instant.from:
  - datetime + Z -> preserve exact time
  - datetime + offset -> "
  - datetime + Z + bracket -> preserve exact time, ignore bracket
  - datetime + offset + bracket -> "
  - throws on a bare datetime string, or datetime + bracket

relativeTo:
  - datetime + bracket -> do what ZonedDateTime.from does
  - datetime + Z + bracket -> "
  - datetime + offset + bracket -> "
  - anything else -> do what PlainDateTime.from does

Closes: #1695
Closes: #1696