Fix string literals with prefix, closes #298 (#299)

* Fix parsing of string literals with prefix

The current solution use '"' as opening sequence for all strings.
It should use different opening sequence depending on prefix.

This fix  corrects this behaviour - original opening is used.

* Add regression tests

Author: filipsajdak

regression-tests/pure2-various-string-literals.cpp2

@@ -0,0 +1,11 @@
+main: () = {
+    s1 := u"u\"";
+    s2 := U"U\"";
+    s3 := u8"u8\"";
+    s4 := L"L\"";
+    s5 := R"(R")";
+    s6 := uR"(uR")";
+    s7 := u8R"(u8R")";
+    s8 := UR"(UR")";
+    s9 := LR"(LR")";
+}

regression-tests/test-results/apple-clang-14/pure2-various-string-literals.cpp.execution

@@ -0,0 +1,23 @@
+
+#define CPP2_USE_MODULES         Yes
+
+#include "cpp2util.h"
+
+#line 1 "pure2-various-string-literals.cpp2"
+auto main() -> int;
+
+//=== Cpp2 definitions ==========================================================
+
+#line 1 "pure2-various-string-literals.cpp2"
+auto main() -> int{
+    auto s1 {u"u\""}; 
+    auto s2 {U"U\""}; 
+    auto s3 {u8"u8\""}; 
+    auto s4 {L"L\""}; 
+    auto s5 {R"(R")"}; 
+    auto s6 {uR"(uR")"}; 
+    auto s7 {u8R"(u8R")"}; 
+    auto s8 {UR"(UR")"}; 
+    auto s9 {LR"(LR")"}; 
+}
+

regression-tests/test-results/apple-clang-14/pure2-various-string-literals.cpp.output

@@ -0,0 +1,11 @@
+main: () = {
+    s1 := u"u\"";
+    s2 := U"U\"";
+    s3 := u8"u8\"";
+    s4 := L"L\"";
+    s5 := R"(R")";
+    s6 := uR"(uR")";
+    s7 := u8R"(u8R")";
+    s8 := UR"(UR")";
+    s9 := LR"(LR")";
+}

regression-tests/test-results/clang-12/pure2-various-string-literals.cpp.execution

@@ -0,0 +1,2 @@
+pure2-various-string-literals.cpp2... ok (all Cpp2, passes safety checks)
+

regression-tests/test-results/clang-12/pure2-various-string-literals.cpp.output

@@ -373,7 +373,9 @@ auto expand_string_literal(
     ++pos;
     auto current_start = pos;   // the current offset before which the string has been added to ret
 
-    auto parts = string_parts{"\"", "\"", string_parts::on_both_ends};
+    auto parts = string_parts{std::string(text.substr(0, current_start)), // begin sequence ", U", u8" depends on the string type 
+                              "\"", // end sequence
+                              string_parts::on_both_ends}; // add opening and closing sequence to generated string
 
     //  Now we're on the first character of the string itself
     for (