`:add module` command at debug repl

[Saizan]

Once PR #306 is merged it could make sense to have a :add module command, like ghci, which imports the inner scope of module.

It's a matter of parsing the command, resolving it to a Module and import it with IIModule.

[alt-romes]

Note: where :add behaves differently to import because import only brings into scope the exported bindings and :add brings all defined bindings.

We should consider additional REPL commands in the greater scheme of things, as in #144 (Debugger Scripting DSL which includes this kind of commands).

There are a couple of different contexts:

1. hdb cli mode: should have the full scriptable language
2. print/eval in cli mode AND the vscode REPL behave similarly, where typing expressions into the REPL or after print means evaluating the Haskell expression/or import statement/binding new variables
3. A third context that allows calling "Debugger commands" (as opposed to Haskell evaluation) from both the cli AND from REPL. This might look like GHCi's commands: a command starting with : in 2. means to go from the 2. context to 3.

Even better, we could consider just having the first two modes:

• 1. goes to 2. by using eval/print
• 2. goes to 1. by using :.

The slight complication with 2. going to 1. is that you'd be able to manipulate the debugger state (e.g. :continue) from the VScode REPL.
That sounds great! But I'm not sure if it's easy or possible to keep in sync the debugger state displayed by the VSCode editor when it is changed by a driver other than the VSCode editor. I suspect this may be possible though! Perhaps even easy, like just sending an invalidated request which retriggers VSCode asking for info.

That would be great!