Allow 1-char base domain

[AstroProfundis]

This PR updates the regrex used to validate base URL and allow 1-char short base domain like x.com for example.

I didn't figured out the meaning of . in http pattern matching group so I've removed it, as well as adjusted the pattern.

I've tested the pattern with https://regex101.com/ and the following test cases:

https://example.com
http://example.com
https://x.com
http://x.com
https://.null
http://.null
https://x.x
http://x.x
https://sth.longer
http://sth.longer
https://sth.reallylong
http://sth.reallylong

And I've also manually updated the plugin installed on my WP and it appears working as expected.

[AstroProfundis]

PS, current pattern allows \-@:%._\+~#= as the first char of domain, I'm not sure if this could also be a problem. I'd guess it might be useful to support basic auth?