Support proof environments

[ScriptRaccoon]

In Markdown files, proofs were previously written "manually", like this:

_Proof._ Let $A$ be a ....
This finishes the proof. <span class="qed">$\square$</span>

This is now replaced by a separate proof environment. It takes care of everything: italicizing "Proof", adding the QED symbol, and handling the spacing.

::: Proof
Let $A$ be a ....
This finishes the proof.
:::

The syntax is similar to that of the existing theorem environments.