Allow splitting string literals into multiple parts

test/libsolidity/syntaxTests/string/string_multipart_only_regular.sol

@@ -0,0 +1,8 @@
+contract test {
+    function f() public pure returns (bytes32) {
+        bytes32 escapeCharacters = "foo" "bar" hex"aa";
+        return escapeCharacters;
+    }
+}
+// ----
+// ParserError: (112-119): Expected ';' but got 'HexStringLiteral'