feat(deps): support upgrading fish itself via cargo install fish

Fish 4.x is a Rust rewrite published to crates.io as 'fish'. Added the
crate to the catalog so fish-deps update will run cargo install --force
fish to upgrade it. A yellow restart reminder is printed after the
upgrade since the new binary won't take effect until the shell restarts.

README warning updated to mention cargo install fish as the upgrade
path for users stuck on Fish 3.x.

functions/_fish_deps_catalog.fish

@@ -25,7 +25,7 @@ function _fish_deps_catalog
         rec rec rec rec rec rec rec rec rec rec rec rec rec rec
 
     set -g _fdc_cargo \
-        "" "" "" starship "" zoxide "" "" \
+        fish "" "" starship "" zoxide "" "" \
         "" "" \
         eza lsd bat "" du-dust "" "" "" ripgrep "" "" trash-cli "" ""