Add social icon of x-twitter

[boykush]

Hello.

This is a minor suggestion, I would like to add an X (twitter) icon to social icons.

If you don't mind, I'll submit a pull request for the same commit as this pull request.

[boykush]

As a side note, the svg file in the pull request is a file downloaded from font awesome in accordance with LICENSE.

Link: https://fontawesome.com/icons/x-twitter?f=brands&s=solid

[not-matthias]

Hey, feel free to create a pull request :)