[FEATURE] Add custom hook support

[TungstnBallon]

Closes #633.

This PR adds hook functionality to the Jayvee interpreter. Hooks are custom functions that are automatically executed before / after blocks. See the PreBlockHook and PostBlockHook types for such a functions signature.

Library users can add hooks with the addHook method:

const program = /* parse program ...*/;

const before: PreBlockHook = (blocktype, input, context) => { /* ... */ };
program.addHook('preBlock', before, { blocking: true });

const after: PostBlockHook = (blocktype, input, context, output) => { /* ... */ };
program.addHook('postBlock', after);  // Options can be omitted. Non-blocking is the default

// interpret model ...