Fix trailing slash handling in directory tests

[mcollard0]

Resolves issue where `z mydir/` fails even when mydir exists.

Shell built-in directory tests (`[ -d path/ ]`) can fail when paths have trailing slashes. This fix strips trailing slashes before the directory test while preserving the original path for cd.

Changes

• Fish: Use string replace to strip trailing slashes before -d test
• Bash: Use `${1%%+(/)}` parameter expansion to remove trailing slashes
• Zsh: Use `${1%%/##}` parameter expansion to remove trailing slashes
• POSIX: Loop to strip trailing slashes preserving root directory
• Tcsh: Use sed to strip trailing slashes before directory test

Key Design Decisions

• Strip only for testing - trailing slashes removed only for directory existence test
• Preserve user intent - original path (with trailing slash) still passed to cd
• Root directory handling - special care to preserve `/` as root directory
• No breaking changes - backward compatible with paths without trailing slashes

[mcollard0]

Initial tests are conclusive. No evidence of cancer is not evidence of no cancer. Further testing by peers is requested.

~/.local/bin/zoxide init fish | source
cd zoxide
zoxide: you are already in the only match

cd /
cd zoxide
cd /
cd zoxide/
pwd
/run/media/michael/FAST_ARCHIVE/Programming/zoxide