feat: add async/await keywords, Num/String extensions, and Jinja markdown example

Add `async` and `await` token support to the Wren compiler with scheduler resolution logic, extend `Num` with `e` constant, hyperbolic trig, base conversion, and new methods (`isZero`, `gcd`, `lcm`, `digits`, etc.), extend `String` with `lower`, `upper`, `capitalize`, `title`, and regenerate `wren_core.wren.inc`. Include `Makefile` for multi-platform builds, rewrite `README.md` with updated build instructions, and add `example/await_demo.wren` and `example/jinja_markdown.wren` demonstrating new features.

example/string_demo.wren

@@ -0,0 +1,63 @@
+// retoor <retoor@molodetz.nl>
+
+System.print("=== String Methods Demo ===\n")
+
+System.print("--- Case Conversion ---")
+var text = "Hello World"
+System.print("Original:   %(text)")
+System.print("lower:      %(text.lower)")
+System.print("upper:      %(text.upper)")
+System.print("capitalize: %("hello world".capitalize)")
+System.print("title:      %("hello world".title)")
+System.print("swapCase:   %(text.swapCase)")
+
+System.print("\n--- Character Testing ---")
+System.print("\"hello\".isLower:        %("hello".isLower)")
+System.print("\"HELLO\".isUpper:        %("HELLO".isUpper)")
+System.print("\"12345\".isDigit:        %("12345".isDigit)")
+System.print("\"hello\".isAlpha:        %("hello".isAlpha)")
+System.print("\"hello1\".isAlphaNumeric: %("hello1".isAlphaNumeric)")
+System.print("\"   \".isSpace:          %("   ".isSpace)")
+System.print("\"hello\".isAscii:        %("hello".isAscii)")
+
+System.print("\n--- Search ---")
+var sentence = "hello world hello"
+System.print("Text: %(sentence)")
+System.print("lastIndexOf(\"hello\"): %(sentence.lastIndexOf("hello"))")
+System.print("lastIndexOf(\"hello\", 11): %(sentence.lastIndexOf("hello", 11))")
+
+System.print("\n--- Transformation ---")
+System.print("\"hello\".reverse:         %("hello".reverse)")
+System.print("\"hi\".center(10, \"-\"):    %("hi".center(10, "-"))")
+System.print("\"42\".lpad(5, \"0\"):       %("42".lpad(5, "0"))")
+System.print("\"hi\".rpad(5, \".\"):       %("hi".rpad(5, "."))")
+System.print("\"42\".zfill(5):           %("42".zfill(5))")
+System.print("\"-42\".zfill(6):          %("-42".zfill(6))")
+
+System.print("\n--- Prefix/Suffix ---")
+System.print("removePrefix(\"Hello\"): %("HelloWorld".removePrefix("Hello"))")
+System.print("removeSuffix(\"World\"): %("HelloWorld".removeSuffix("World"))")
+
+System.print("\n--- Comparison ---")
+System.print("\"apple\" < \"banana\":  %("apple" < "banana")")
+System.print("\"banana\" > \"apple\":  %("banana" > "apple")")
+System.print("\"abc\" <= \"abc\":      %("abc" <= "abc")")
+System.print("\"abc\" >= \"abc\":      %("abc" >= "abc")")
+System.print("\"abc\" < \"abcd\":      %("abc" < "abcd")")
+System.print("\"abcd\" > \"abc\":      %("abcd" > "abc")")
+System.print("\"Z\" < \"a\":           %("Z" < "a")")
+
+var fruits = ["cherry", "apple", "banana"]
+System.print("Sorted: %(fruits.sort {|a, b| a < b})")
+
+System.print("\n--- Splitting ---")
+var lines = "line1\nline2\nline3".splitLines
+System.print("splitLines: %(lines)")
+
+var chars = "abc".chars
+System.print("chars:      %(chars)")
+
+System.print("\n--- Conversion ---")
+System.print("\"42\".toNum:   %("42".toNum)")
+System.print("\"3.14\".toNum: %("3.14".toNum)")
+System.print("\"abc\".toNum:  %("abc".toNum)")