fix: reject multiple patterns when -P/--perl-regexp is used

GNU grep's PCRE backend supports only a single pattern. Supplying
multiple patterns via repeated -e flags, or a pattern string that
contains a literal newline, must exit 2 with the message:

    the -P option only supports a single pattern

Add the validation immediately after patterns are collected, before
regex-mode selection. Add a test covering:
  - two separate -e flags with -P
  - a newline-embedded pattern string with -P
  - single -e with -P still works normally

Closes #34

src/lib.rs

@@ -255,6 +255,14 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         ));
     }
 
+    // GNU grep's PCRE backend (-P) supports only a single pattern.
+    if perl_regexp && patterns.len() > 1 {
+        return Err(USimpleError::new(
+            2,
+            "the -P option only supports a single pattern".to_string(),
+        ));
+    }
+
     // Decoded options into enums
     let regex_mode = if fixed_strings {
         RegexMode::Fixed