Add Zsh support

[chenyao36]

Important diff to support zsh:

-if [ "$(jobs)" == "" ]
+if [[ "$(jobs)" = "" ]]

-    ENVRC_LOAD="{rc_found}" ENVRC_PPID=$$ $BASH
+    ENVRC_LOAD="{rc_found}" ENVRC_PPID=$$ $SHELL

Other diff:

• change: envrc bash -> envrc hook;
• new: envrc init bash and envrc init zsh;
• refactor: cargo fmt; clap 2 builder -> clap 4 derive;

[runlevel5]

LGTM 👍

[roxma]

This PR breaks existing config. I don't want to merge it.