Add syntax tests to augment the test coverage of

libsolidity/parsing/Scanner.cpp and libsolidity/parsing/Scanner.h
Fix #4627 and PR #5003.

- Add multiline comment test
- Add upper case hex literal test
- Add test for unicode escapes
- Add test for strings with escaped newlines
- Add test for string escapes
- Add test for strings that do not terminate before end of file
- Add test for unterminated blocks

test/libsolidity/syntaxTests/unterminatedBlocks/zero_dot_x.sol

@@ -0,0 +1,5 @@
+contract test {
+    function f() pure public { 0.x; }
+}
+// ----
+// TypeError: (47-50): Member "x" not found or not visible after argument-dependent lookup in int_const 0.