feat: add matrix to social icons

[christianfosli]

Adds the matrix icon to social icons.

Matrix is a decentralized instant messaging protocol. It works similar to email but for instant messaging.

I verified that it works with the following config:

socials = [
    { name = "matrix", url = "https://matrix.to/#/@christianfosli:matrix.org", icon = "matrix" },
]

[not-matthias]

Thanks!