chore: rename test files to snake_case and update string class docs

Rename indexOf.wren and startsWith.wren test files to index_of.wren and
starts_with.wren respectively, fix missing trailing newlines, update
string class documentation to clarify UTF-8 storage and move the
note about incorrect UTF-8 handling in the index operator, and
remove unnecessary parentheses in string_indexOf native return.

src/wren_core.c

@@ -955,7 +955,7 @@ DEF_NATIVE(string_indexOf)
 
   char* firstOccurrence = strstr(string->value, search->value);
 
-  RETURN_NUM(firstOccurrence ? (firstOccurrence - string->value) : -1);
+  RETURN_NUM(firstOccurrence ? firstOccurrence - string->value : -1);
 }
 
 DEF_NATIVE(string_startsWith)