add subscript extension

[zkamvar]

This implements a subscript extension for the single tilde. I would like help working this into a more robust patch format.

I also note that this has the problem where both subscript and strikethrough use ~ as matching, so when they are both enabled, then the subscript markup is ignored and I'm not sure how to handle that.

commonmark::markdown_commonmark("H~~2~~O and H~2~O", extensions = c("strikethrough", 
    "subscript"))
#> [1] "H~~2~~O and H\\~2\\~O\n"

^{Created on 2024-10-17 with reprex v2.1.1}