Backwards compatibility check does not check for declare or export

[gbtec-markusmeier]

Check list

• [x] I have read through the README
• [x] I have the latest version of forgit
• [x] I have searched through the existing issues

Environment info

• OS
  • [x] Linux
  • [ ] Mac OS X
  • [ ] Windows
  • [ ] Others:
• Shell
  • [x] bash
  • [ ] zsh
  • [ ] fish

Problem / Steps to reproduce

When loading forgit, backwards compatilibility checks reports variables not being exported although they are.

Reason is that bash's export does not return FORGIT_PAGER="less", but:

• declare -x FORGIT_PAGER="less" in default mode
• export FORGIT_PAGER="less" in posix mode (set -o posix)

We should additionally check for export in https://github.com/wfxr/forgit/blob/master/forgit.plugin.zsh#L25.

Checking for declare -x won't be necessary when we fix https://github.com/wfxr/forgit/issues/256.

$ bash --version
GNU bash, version 5.1.16(1)-release (x86_64-pc-linux-gnu)