refactor(scripts): add set -euo pipefail to all installer scripts

Add strict error handling to all scripts:
- 13 scripts get `set -euo pipefail`
- install-macos-defaults.sh gets `set -uo pipefail` (without -e) because
  defaults write commands may fail on newer macOS versions
- install-cargo-packages.sh: also add missing source of shared.sh

scripts/create-nvim-keymaps.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+set -euo pipefail
 # @description Create file containing key mappings for Neovim
 # Usage: ./create-nvim-keymaps.sh
 #