feat: add zoxide env command

[buiducnhat]

Summary

Adds a new zoxide env subcommand that prints the resolved values of zoxide's environment variables after defaults have been applied, as proposed in #803.

Since default environment variables vary across platforms and require processing in Rust, this provides a useful angle when debugging configuration issues.

Usage

Print all variables (NAME=VALUE, one per line):

$ zoxide env
_ZO_DATA_DIR=/home/user/.local/share/zoxide
_ZO_ECHO=
_ZO_EXCLUDE_DIRS=/home/user
_ZO_FZF_OPTS=
_ZO_MAXAGE=10000
_ZO_RESOLVE_SYMLINKS=

Print a single variable (raw value, for scripting):

$ zoxide env _ZO_DATA_DIR
/home/user/.local/share/zoxide

Notes

• Values come directly from the existing functions in config.rs, so the printed output always matches runtime behavior (e.g. _ZO_DATA_DIR falls back to the platform data dir, _ZO_EXCLUDE_DIRS falls back to the home directory glob, _ZO_MAXAGE defaults to 10000).
• Unset variables with no implicit default (_ZO_ECHO, _ZO_FZF_OPTS, _ZO_RESOLVE_SYMLINKS) are printed with an empty value.
• Shell completions (all shells) are regenerated via build.rs; a zoxide-env(1) manpage is included and linked from zoxide(1).

Closes #803