json2: make decoder2 fns public

[gechandesu]

2d33a7f breaks programs that already uses x.json2.decoder2 module. This PR fixes it.

main.v:173:15: warning: function `x.json2.decoder2.decode` has been deprecated since 2025-10-12, it will be an error after 2026-04-10; `decode` has been moved to `x.json2`, use `decode` from `x.json2` instead
  171 |     j := json.encode[item.Item](i, prettify: true, indent_string: '  ')
  172 |     println(j)
  173 |     d := decoder.decode[item.Item](j)!
      |                  ~~~~~~~~~~~~~~~~~~~~
  174 |     assert i == d
  175 | }
main.v:173:15: error: function `x.json2.decoder2.decode` is private
  171 |     j := json.encode[item.Item](i, prettify: true, indent_string: '  ')
  172 |     println(j)
  173 |     d := decoder.decode[item.Item](j)!
      |                  ~~~~~~~~~~~~~~~~~~~~
  174 |     assert i == d
  175 | }

[huly-for-github]

Connected to Huly®: V_0.6-26180

[spytheman]

Thank you 🙇🏻 .