Spec: Fix algorithm declarations (#147)

The Bikeshed Documentation (dated 8 March 2024) says, "Algorithms can be
explicitly indicated in your markup by putting the algorithm attribute
on a container element or a heading" [1].

Through trial and error, I discovered that algorithm-specific markup is
not generated when you write <dfn algorithm>. Instead, the <dfn> tag
must be wrapped in something like <div algorithm>.

This change has at least two positive results:

1. Local variables are no longer declared in the global scope!
2. We can now click on local variables and they are highlighted in
   pretty colors.

[1]: https://speced.github.io/bikeshed/#var-and-algorithms