Accept era years of three or more digits in Date._jisx0301

[Shaisolaris]

Date#jisx0301 writes the era year without a limit on its width, so dates from 2118 on (Reiwa 100) come out with three digits, but Date._jisx0301 only accepts two digits there. The output of #jisx0301 then cannot be read back:

d = Date.new(2118, 6, 1)
d.jisx0301                 # => "R100.06.01"
Date.jisx0301(d.jisx0301)  # => Date::Error: invalid date
Date._jisx0301("R100.06.01") # => {}
Date._parse("R100.06.01")    # => {:year=>2118, :mon=>6, :mday=>1}

This change accepts three or more digits for the era year when an era initial is present. Without an initial, the year still has to be two digits, so a string like 100.05.01 is still rejected.

Tests:

• Added cases to test__jisx0301 for R100.05.01, R100.05.01T04:05:06+09:00, 100.05.01 and a Date.jisx0301(d.jisx0301) round trip. They fail on master and pass with this change.
• A round-trip check of iso8601, rfc3339, xmlschema, rfc2822, httpdate and jisx0301 over 100,000 random dates in years 1 to 9999 now passes for all of them. Before the change, jisx0301 failed for every date from 2118 on.
• test/date/test_*.rb locally (Ruby 3.3.8): the failures and errors I see are the same on master and with this change (test__plus__ex, test_new_offset, and tests that need assert_warning, assert_ractor and similar helpers from ruby core that my setup does not load). I did not run the CI matrix.