Add support for breaking on exceptions and pausing R at any time

[lionel-]

Branched from #1033 (for test infra)
Addresses posit-dev/positron#11797
Addresses posit-dev/positron#11799

Adds support for the DAP feature of exception breakpoints, and for pausing the debugger at any time. Both features rely on:

• Calling browser() from a global handler for the relevant condition (error, warning, or interrupt)

• Skipping the top-level frame which points to our global handlers.

  When these handlers trigger, the R stack includes the handler frame at the top. Since this frame is not useful to users, we track why we entered the debugger via DebugStoppedReason (Condition for errors/warnings, Pause for interrupts) and discard the top frame in start_debug() for these cases.

  Note that the console will still evaluate in this frame until we fix IDE alternative to options(error = recover) positron#9156 (comment).

  Also note that for errors and warnings, we'd ideally automatically skip the part of the call stack involved in emitting/handling the condition (partly the base emitting context with withRestart() etc, and partly the package-level emitting context, e.g. based on rlang's error_call = parent.frame() pattern).

The condition breakpoints are surfaced like this in the UI:

We could add messages as well but I thought that'd crowd the UI, and it doesn't feel as useful to easily break on those. Could be added as a user setting in the future.

Pausing can be done by calling the existing VS Code command "Pause":