add icon for modrinth.com

[btwonion]

Requirements

Go over all the following points, and put an x in all the boxes that apply.

• [x] My issuer icon is fully vector and does not contain (parts of) a JPG/PNG/etc.
• [x] My issuer icon contains the original brand logo, not that from an icon pack.
• [x] My issuer icon is somewhat square, so it's nicely visible in the app.
• [x] My issuer icon starts and ends with the <svg> element.
• [x] My issuer icon is scalable (it uses viewBox instead of a static width/height).
• [x] My issuer icon does not contain whitespace around the SVG (this JSFiddle could help to remove whitespace).
• [x] My issuer icon does not include the doctype element.
• [x] My issuer icon directory and file name is lowercase.
• [x] My issuer icon directory and file are in the vectors/[domain name]/ folder.
• [x] My pull request contains just one icon.