HTML metainfo database

This provides metainformation about HTML elements for correctness checking, and is used by the ahtml crate.

The database about the HTML elements is linked as static structs into the binary, from includes/static_meta_db.rs, which was generated from json files copied from the html / html-sys crates, https://github.com/yoshuawuyts/html.

Static database rebuild

These json files are currently in the ahtml crate (but in the same Git repository as this crate), in the resources/merged/elements/ directory.

If you need to regenerate the static_meta_db.rs file, currently this hacky way is how it's done (due to be replaced with something sane): from the Git repository root:

WRITE_STATIC_META_DB_RS_PATH=libs/ahtml_html/includes/static_meta_db.rs HTML_READ_META_DB_FROM_JSON_DIR=libs/ahtml/resources/merged/elements/ HTML_META_DEBUG=true SESSIONID_HASHER_SECRET=1234 cargo run --bin website

then ctl-c and commit.