Update mime type to `text/vnd.subtext`

[gordonbrander]

Background on MIME types...

• Prior art: Markdown was previously text/x-markdown, then got standardized to text/markdown https://stackoverflow.com/questions/10701983/what-is-the-mime-type-for-markdown
• However, the x- convention was deprecated https://www.rfc-editor.org/rfc/rfc6648
• Instead, MIME introduced subtypes: standard (no prefix), vendor (vnd. prefix), personal or vanity (prs. prefix), unregistered (x. prefix). See https://www.rfc-editor.org/rfc/rfc6838#section-3.4.
• x. is discouraged. vnd. is encouraged, including for noncommercial projects. Apparently they have streamlined the registration/standardization path for vnd.