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":

QA Notes

This is comprehensively tested on the backend side.

Exception breakpoints (break on error/warning):

• In the Debug Console's breakpoint picker, enable "Uncaught Errors" or "Warnings".

• Run code that triggers an error (e.g. stop("oops")) or warning (e.g. warning("hmm")). The debugger should pause at the point of the condition.

• The call stack should show user code (for errors or warnings thrown from C) or stop()/warning(), not R's internal condition handling frames (.handleSimpleError, doWithOneRestart, etc.).

• The exception type and message should appear in the editor and the debug UI (e.g. "simpleError: oops").

• Continue (c) should resume execution and propagate the error/warning normally.

Pause:

• While R is executing a long-running operation (e.g. Sys.sleep(100), an infinite loop, or I/O), run the Debug: Pause command from the command palette. R should interrupt and drop into the debugger at the current execution point.

• The call stack should reflect where R was interrupted.

• Q should exit the debugger. c / Continue should resume running code. Note that with curl downloads or I/O there are known issues.

[lionel-]

I've added some filtering of condition handling/emitting frames, like .handleSimpleError, .signalSimpleWarning, doWithOneRestart, etc