Add optional `href` verifier

[numpad]

Hardcoding links/hrefs is fine, but there should be an optional feature to ensure (internal) links are not dead.

Consider the following scenario:

• index.lua contains a hyperlink to /contact/ (→ contact.lua).
• some time later, the file contact.lua is renamed to contact-us.lua.
• however, the hyperlink in index.lua was never updated and is dead now.

With some kind of validation in place, this should fail during build, and print an error.

a {
  "Contact Us",
  href=luatml_link('contact.lua'),
}