Alternative scheme support for the markdown URL

### Current Behavior

I have following issue with Markdown parser:
```
[alt](gemini://site.com)
```
* where `gemini` is the alternative [protocol](https://geminiprotocol.net); the URL is valid anyway, but could not be parsed with engine (v2 beta)

### Steps to Reproduce

Just text entry in the new message form

### Expected Behavior

It should be clickable, but I see the plain markdown text like on example above

### Screenshots

_No response_

### Environment

- Flarum version: v2 beta (repository)
- Webserver: nginx
- Hosting environment: vps
- PHP version: 8.2

### Output of `php flarum info`

```
php flarum info
Flarum core: 2.0.0-beta.3
PHP version: 8.2.29
SQLite version: 3.40.1
Loaded extensions: Core, date, libxml, openssl, pcre, zlib, filter, hash, json, pcntl, random, Reflection, SPL, session, standard, sodium, PDO, xml, apcu, calendar, ctype, curl, dom, mbstring, FFI, fileinfo, ftp, gd, gettext, iconv, intl, exif, pdo_sqlite, Phar, posix, readline, shmop, SimpleXML, sockets, sqlite3, sysvmsg, sysvsem, sysvshm, tokenizer, xmlreader, xmlwriter, xsl, zip, Zend OPcache
+----------------------+---------+--------+
| Flarum Extensions | | |
+----------------------+---------+--------+
| ID | Version | Commit |
+----------------------+---------+--------+
| flarum-tags | 2.x-dev | |
| flarum-suspend | 2.x-dev | |
| flarum-subscriptions | 2.x-dev | |
| flarum-sticky | 2.x-dev | |
| flarum-mentions | 2.x-dev | |
| flarum-markdown | 2.x-dev | |
| flarum-lock | 2.x-dev | |
| flarum-likes | 2.x-dev | |
| flarum-lang-english | 2.x-dev | |
| flarum-flags | 2.x-dev | |
+----------------------+---------+--------+
Base URL:
Installation path: /var/www/flarum
Queue driver: sync
Session driver: file
Mail driver: mail
Debug mode: off
```

### Possible Solution

_No response_

### Additional Context

_No response_