Make anything adding to .zshrc or .bash_profile or .bashrc idempotent

[karlhorky]

idempotent = can be run multiple times without leading to duplicate lines in like this:

# pnpm
export PNPM_HOME="/Users/omerszerer/Library/pnpm"
case ":$PATH:" in
  *":$PNPM_HOME:"*) ;;
  *) export PATH="$PNPM_HOME:$PATH" ;;
esac
# pnpm end

export PGDATA=/opt/homebrew/var/postgresql@16
export PSQL_PAGER="less --chop-long-lines --header 1"

export PGDATA=/opt/homebrew/var/postgresql@16
export PSQL_PAGER="less --chop-long-lines --header 1"
export PATH="/opt/homebrew/opt/postgresql@16/bin:$PATH"

export PGDATA=/opt/homebrew/var/postgresql@16
export PSQL_PAGER="less --chop-long-lines --header 1"