coffee.plugin.zsh: improve CoffeeScript compilation utilities

Features:

- Add check for CoffeeScript compiler installation
- Add input validation for all functions
- Add proper error messages with usage information
- Add clipboard operation feedback

Improvements:

- Replace aliases with proper functions for better control
- Use local variables to prevent namespace pollution
- Add proper quoting and option handling
- Add error output redirection
- Export functions for subshell availability

The previous implementation lacked error checking and could fail silently
in various scenarios. This update makes the functions more robust and
user-friendly while maintaining the original functionality.

plugins/coffee/coffee.plugin.zsh

@@ -1,16 +1,58 @@
 #!/bin/zsh
 
-# compile a string of coffeescript and print to output
+# Check if coffee is installed
+if ! command -v coffee >/dev/null 2>&1; then
+    echo "Error: CoffeeScript compiler not found. Please install it with 'npm install -g coffee-script'" >&2
+    return 1
+fi
+
+# Compile a string of CoffeeScript and print to output
 cf() {
-  coffee -peb "$1"
+    if [[ -z "$1" ]]; then
+        echo "Error: No input provided" >&2
+        echo "Usage: cf 'CoffeeScript code'" >&2
+        return 1
+    fi
+    coffee -peb -- "$1" 2>/dev/null || {
+        echo "Error: Failed to compile CoffeeScript" >&2
+        return 1
     }
-# compile & copy to clipboard
+}
+
+# Compile & copy to clipboard
 cfc() {
-  cf "$1" | clipcopy
+    if [[ -z "$1" ]]; then
+        echo "Error: No input provided" >&2
+        echo "Usage: cfc 'CoffeeScript code'" >&2
+        return 1
+    fi
+    local result
+    result=$(cf "$1") || return 1
+    echo "$result" | clipcopy && echo "Compiled code copied to clipboard"
 }
 
-# compile from clipboard & print
-alias cfp='cf "$(clippaste)"'
+# Compile from clipboard & print
+cfp() {
+    local input
+    input=$(clippaste)
+    if [[ -z "$input" ]]; then
+        echo "Error: Clipboard is empty" >&2
+        return 1
+    fi
+    cf "$input"
+}
 
-# compile from clipboard and copy to clipboard
-alias cfpc='cfp | clipcopy'
+# Compile from clipboard and copy back to clipboard
+cfpc() {
+    local input result
+    input=$(clippaste)
+    if [[ -z "$input" ]]; then
+        echo "Error: Clipboard is empty" >&2
+        return 1
+    fi
+    result=$(cf "$input") || return 1
+    echo "$result" | clipcopy && echo "Compiled code copied to clipboard"
+}
+
+# Export the functions
+export -f cf cfc cfp cfpc