feat: Watch expressions

[alt-romes]

What are the features supported by the VSCode/DAP watch expressions or conditional breakpoints?

Investigate the potential capabilities and implement them to make sure the Watch variables pane can be used.

[alt-romes]

This is what Watch Variables when debugging javascript looks like:

• Variables are shown as variables like those in the variables pane
• Expressions are evaluated everytime we break

When the variable is out of scope, a runtime error for the given expression appears:

The current behavior of the Haskell debugger is to

• Print variables as strings rather than show the structured format
• Evaluate all expressions normally every breakpoint
• Show runtime error that variable is no longer in scope when the scope is exited

So it seems we're already doing most things correctly. I do agree with JS here that it would be better for the variable to be structured and expandable if it is a variable rather than an expression.

[alt-romes]

In essence, this ticket can be closed when #116 is fixed.