Use `hdb cli` mode in DAP REPL

[alt-romes]

When we're stopped at a breakpoint, we can use a REPL (e.g. in VSCode) to evaluate expressions.

However, that is a bit limited in that it only prints expressions, while being useless in controlling the debugger.

I propose to make the REPL in DAP mode make available the full cli mode (as in, the standalone hdb mode).
This will allow the user to still print expressions (with print <expr> or p <expr>), but also do more complex tasks such as starting programs and defining variables.

This mimics the behavior of e.g. the Xcode debugger, where a breakpoint in a Swift program will have the high level utilities with a side panel for lldb commands.

[alt-romes]

When the DAP repl invalidates the current breakpoint (e.g. by stepping next), the HDB server must ensure the GUI is refreshed to reflect the new stopped state. I'm sure there's someway to do this within DAP, at least similar to the way we refresh variables after one is forced.