refactor(envs): `GHJK_NEXTFILE` based env reloading

[Yohe-Am]

Describe your change

• Nextfile mechanism to improve sync and activate experience.
  • Reload hooks set GHJK_NEXTFILE to a temporary path based on the shell's PID
  • When sync or activate is invoked, the next env to activate will be written to nextfile path (if var detected)
  • If no nextfile var is detected, a new shell is launched
  • Reload hooks check if nextfile on every postexec and if found, will activate env name within
• Improves readability of path strings in activate scripts

Motivation and context

MET-502, MET-503

Migration notes

N/A

Checklist

• [x] The change come with new or modified tests
• [x] Hard-to-understand functions have explanatory comments
• [ ] End-user documentation is updated to reflect the change

[linear[bot]]

MET-502 (ghjk) fix the shell loading issue