feat: add \U style 8-hex-digit unicode escape support in string literals

Extend the compiler's string parser to handle `\U` escapes with 8 hex digits
for codepoints above U+FFFF, replacing the previous TODO comment. The
`readUnicodeEscape` helper now accepts a variable digit count (4 or 8) and
the `readString` function dispatches `\u` with length 4 and `\U` with length 8.
New test cases verify correct encoding of emoji and ancient scripts, plus
error handling for incomplete long escapes and values that fit in 4 digits.

test/language/string/incomplete_long_unicode_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\U01F603"

test/language/string/unicode_escapes.wren

@@ -14,4 +14,11 @@ System.print("\u0b83")     // expect: ஃ
 System.print("\u00B6")     // expect: ¶
 System.print("\u00DE")     // expect: Þ
 
-// TODO: Syntax for Unicode escapes > 0xffff?
+// Big escapes:
+var smile = "\U0001F603"
+var byteSmile = "\xf0\x9f\x98\x83"
+System.print(byteSmile == smile) // expect: true
+
+System.print("<\U0001F64A>")       // expect: <🙊>
+System.print("<\U0001F680>")       // expect: <🚀>
+System.print("<\U00010318>")       // expect: <𐌘>

test/language/string/unicode_two_bytes_to_long_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\U0060"