Add standalone MathML files in MML format

[Schweinepriester]

• Inspired by https://mastodon.social/@pepelsbey/113233014550351693
• Spec:
  • https://w3c.github.io/mathml/#encoding-names
  • https://www.w3.org/TR/MathML2/chapter7.html#id.7.1.3
  • https://w3c.github.io/mathml-docs/mathml-media-types/#media-types-mathml
• Supported in Firefox
• Chrome: https://issues.chromium.org/u/0/issues/370694822
• Safari: https://bugs.webkit.org/show_bug.cgi?id=280682
• Test: Could be copied from the toot