Implement `catch` expression

[katsaii]

What is your feature request?

Implement new catch keyword for exception handling.

Please describe in detail how you expect this new feature to behave.

There will be no try keyword, just catch:

let result = potentiallyFailingExpression() catch {
  defaultResultIfAnExceptionOccurs();
};

GML-like try/catch blocks can be written in Catspeak like:

do {
  potentiallyFailingExpression();
} catch ex {
  show_message("failed! {0}", ex);
}