feat: csv: new timezone rule; convert zoned date-times to local dates

[simonmichael]

Previously, CSV date-times with a different time zone from yours (with or without explicit timezones in the CSV) could give off-by-one dates, because the CSV timezone was ignored.

Now,

1. you can use the timezone rule to indicate which other timezone a CSV is implicitly using

2. CSV date-times with a timezone - whether declared by rule or parsed with %Z - are localised to the system time zone (or another set with the TZ environment variable).

[crocodile-code-review]

Review on Crocodile

[simonmichael]

Mail list discussion: https://groups.google.com/g/hledger/c/P3tYKCfjKfA/m/AfGUGuJFBQAJ

Docs:

timezone

timezone TIMEZONE

When CSV contains date-times that are implicitly in some time zone
other than yours, but containing no explicit time zone information,
you can use this rule to declare the CSV's native time zone,
which helps prevent off-by-one dates.

When the CSV date-times do contain time zone information,
you don't need this rule; instead, use %Z in date-format
(or %z, %EZ, %Ez; see the formatTime link above).

In either of these cases, hledger will do a time-zone-aware conversion,
localising the CSV date-times to your current system time zone.
If you prefer to localise to some other time zone, eg for reproducibility,
you can (on unix at least) set the output timezone with the TZ environment variable, eg:

$ TZ=HST hledger print -f foo.csv  # or TZ=HST hledger import foo.csv