This implements the core memory "model" for debugging that reflects
DAP, in `debugModel.ts`. It also implements a filesystem provider based
on that in `debugMemory.ts`, for tentative application in the hex editor.

Finally it adds context menu items for these. This works with changes in
mock debug, but for some reason reopening the ".bin" file in the hex
editor results in a blank editor. Still need to look at that.

Ultimately though, as indicated in #126268, we'll probably want custom
commands for the hex editor to call as low level read/write is not
supported in the stable API. Also, the file API doesn't represent the
"unreadable" ranges which DAP supports.

…-memory