Handle invalid string literals.

test/string/incomplete_unicode_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\u00"

test/string/incomplete_unicode_escape_at_eof.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\u004

test/string/invalid_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"not a \m real escape"

test/string/invalid_unicode_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\u00no"

test/string/unterminated.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"this string has no close quote