LibRegex: Display correct position for Compare in REGEX_DEBUG

When REGEX_DEBUG is enabled, LibRegex dumps a table of information
regarding the state of the regex bytecode execution. The Compare opcode
manipulates state.string_position directly, so the string_position value
cannot be used to display where the comparison started; therefore, this
patch introduces a new variable to keep track of where we were before
the comparison happened.

Userland/Libraries/LibRegex/RegexMatch.h

@@ -256,6 +256,7 @@ struct MatchInput {
 };
 
 struct MatchState {
+    size_t string_position_before_match { 0 };
     size_t string_position { 0 };
     size_t instruction_position { 0 };
     size_t fork_at_position { 0 };