Fix parsing of string literals that contains //, closes #442 (#444)

In the following code:
```cpp
main: () = {
	{ std::cout << "//456"; } //error

	:()->_ = { return "//Error"; };

	b:="/* error even without {} block";
}
error: (x) -> _ = { return "//"; }
```
`//` and `/*` inside string literals are parsed as begining of comments.

Current change solves that. The missing is handling of raw string literals.

Closes #442

Author: filipsajdak

source/io.h

@@ -693,13 +693,20 @@ auto process_cpp2_line(
     auto found_end = false;
 
     auto prev = ' ';
+    auto in_string_literal = false;
+
     for (auto i = colno_t{0}; i < ssize(line); ++i) {
 
         if (in_comment) {
             switch (line[i]) {
             break;case '/': if (prev == '*') { in_comment = false; }
             break;default: ;
             }
+        } else if (in_string_literal) {
+            switch (line[i]) {
+            break;case '"': if (prev != '\\') { in_string_literal = false; }
+            break;default: ;
+            }
         }
 
         else {
@@ -732,6 +739,9 @@ auto process_cpp2_line(
             break;case '/':
                 if (prev == '/') { in_comment = false; return false; }
 
+            break;case '"':
+                if (prev != '\\') { in_string_literal = true; }
+
             break;default: ;
             }
         }