add test file for wide string constants

add full UTF-8 to wide-str language capabilities
move wstr language constant expressions to builtins and adjust test cases

test/unit/regression/wstring.c3

@@ -0,0 +1,28 @@
+module std::string::wstring_test;
+import std;
+
+WString a = @wstring("Hellø");
+Char16[] x = @char16("Är du där?");
+Char32[] x2 = @char32("你好");
+
+fn void sanity_checks() @test
+{
+	test::eq(a[:6], (Char16[]) { 'H', 'e', 'l', 'l', 'ø', 0 });
+	test::eq(x, "Är du där?".to_temp_utf16()!!);
+	test::eq(x2, "你好".to_temp_utf32()!!);
+}
+
+fn void test_get_value() @test
+{
+	test::eq((ushort)'Ä', $$wstr16("ÅÄÖ")[1]);
+	const uint VAL = $$wstr32("XYZ👍ABC")[3];
+	test::eq((uint)'👍', VAL);
+}
+
+fn void lengths() @test
+{
+	test::eq($$wstr16("ABC").len, 4);
+	test::eq($$wstr16("ABC", false).len, 3);
+	Char16[?] w = $$wstr16("ÅÄÖ");
+	test::eq(w, (Char16[?]) { 'Å', 'Ä', 'Ö', 0 });
+}