Add controls for shorthand

[jdutant]

Each (document-level) theorem kind should have a shorthand option that allows the user to enter theroem of that kind via a shorthand. Use cases:

• a user could specify that statements of a certain kinds are jokes and enter them as follows:

::: joke
Decimals have a point.
:::

• perhaps some users may want to use our predefined styles but deactivate their shortands, so that the following isn't interpreted as a mathematical proposition?

::: proposition
Decimals have a point.
:::

But that at the same time they could still write:

::: {.statement kind=proposition}
Substracting the same from the same yields the same.
:::

Proposal: add a property shorthand to statement kinds. Question: should that be a switch (yes / no, and the shorthand is the kind name?) or a string (so that user could set up jk as a shorthand for joke)?

I favour the switch.