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

[doZennn]

Description
Allow multiple characters as a mention symbol.

Example
Currently "symbol" can only be a single character. Would be great if we could use more than one character like so:

$config = [
    'mentions' => [
        'user_url_generator' => [
            'symbol'    => 'u:',
            'regex'     => '/^[a-z0-9]+/i',
            'generator' => $userMentionGenerator,
        ],
    ]
];

[markhalliwell]

This, currently, isn't possible and will definitely require something outlined like in #492 (comment)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.