register new context menu entries

[GitMensch]

it would be nice to be able to:

• possibly register the navigation, if the active COBOL extension supports it (bitlang.cobol has a "go to definition" that may be usable for example)
• the view group would could be filled with "show data" (which is done via libcob as is now, if available) with "show as hex" (check for null value, then asking GDB to p/x var->data) and "show as binary" (check for null value, then asking GDB to p/t var->data)
• add to watch (=adding to the watch list) and "break on value changes" (=gdb watch of var->data) should be available, too

See microsoft/vscode#70377 for details of this new feature that is available in insider and on the list for the September 2020 release.