treewide: add 'shellcheck shell' directives

I used 'bash' for zsh files, since that's the closest and `shell=zsh` is
not supported [1].

[1]: https://github.com/koalaman/shellcheck/issues/809

bootstrap.sh

@@ -1,5 +1,6 @@
 #!/usr/bin/env nix-shell
 #! nix-shell -i bash -p bitwarden-cli git gnupg jq nix
+# shellcheck shell=bash
 
 # Command failure is script failure
 set -e

home/zsh/completion-styles.zsh

@@ -1,3 +1,5 @@
+# shellcheck shell=bash
+
 # Style the completion a bit
 zstyle ':completion:*' list-colors ${(s.:.)LS_COLORS}
 # Show a prompt on selection

home/zsh/extra-mappings.zsh

@@ -1,3 +1,4 @@
+# shellcheck shell=bash
 # shellcheck disable=2154
 
 # Fix Ctrl+u killing from the cursor instead of the whole line

home/zsh/options.zsh

@@ -1,3 +1,5 @@
+# shellcheck shell=bash
+
 # Show an error when a globbing expansion doesn't find any match
 setopt nomatch
 # List on ambiguous completion and Insert first match immediately