Allow more than one character to be used as a symbol for MentionExtension

[colinodell]

Thanks to #549 we're able to implement this for the upcoming 2.0 release

Resolves #514

[markhalliwell]

I feel like both the symbol and mention regex should be combined into a single regex that uses parenthesis to group the individual results. Maybe something like:

        return InlineParserMatch::join(
          InlineParserMatch::string($this->symbol),
          InlineParserMatch::regex($this->mentionRegex),
        );

Unfortunately, this would also mean that string $match would have to really be array $matches.

Also, is there a reason we can't add both of these to InlineParserContext, i.e. $inlineContext->getMatch(): string and $inlineContext->getMatches(): array; the former just returnining the first item of the latter?

Then we could something like the following here:

        // Extract the symbol and identifier from the matches.
        [$symbol, $identifier] = $inlineContext->getMatches();

        // Advance past the symbol to keep parsing simpler
        $cursor->advanceBy(\mb_strlen($symbol));

[colinodell]

Great idea! I've incorporated this into #560.