[Debugger] Treat user variable filter settings as a mergable add-on

[afshin]

When the user sets variableFilters in their own user settings, these should be merged with the defaults instead of overwriting them. If this behavior is something that users object to, we can add a boolean to set whether the behavior is to merge or the overwrite in the future.

[JohanMabille]

I think most of the current defaults should actually be moved to the backend, because there are kernel specific.
We could add a field in the debug_info_request listing these variables so they can be merged with those set by the user.

[jtpio]

It's also worth noting that debugpy now puts some of the original filtered variables (which was not the case with ptvsd):

jupyterlab/packages/debugger-extension/schema/main.json

Lines 39 to 52 in b13cc05

	"debugpy",
	"display",
	"get_ipython",
	"ptvsd",
	"_xpython_get_connection_filename",
	"_xpython_launch",
	"_pydev_stop_at_break",
	"__annotations__",
	"__builtins__",
	"__doc__",
	"__loader__",
	"__name__",
	"__package__",
	"__spec__"

Under special variables:

[krassowski]

Slightly off-topic, but just to spell this out, it appears that the defaults for xpython are now missing:

"_",
"quit",
"exit",
"_i",
"_ii",
"_iii",
"_i1",
"_i2",
"_i3",
"_ih",
"_oh",
"_dh",
"Out",
"In"

which was not the case for older versions of xeus-python (I believe it has to do with the rework of xeus-python to be more closely tied to IPython). I might be wrong though, I don't know a lot about the debugger.

Could these filters become regular expressions? Currently it seems that specifying "_i\d" is not possible, but it could help to hide all (IPython or not) input history. There are some confused users out there on SO.

[jtpio]

I believe it has to do with the rework of xeus-python to be more closely tied to IPython

It's mostly related to the switch from ptvsd to debugpy, and also some other refactors in xeus-python to use IPython directly (jupyter-xeus/xeus-python#400).

Could these filters become regular expressions

For sure. For now the check is happening in:

jupyterlab/packages/debugger/src/panels/variables/tree.tsx

Lines 119 to 121 in d895f35

	?.filter(
	variable => !(filter || new Set()).has(variable.evaluateName || '')
	)

So this logic could already be updated to test against regexes instead.