Add global snippets loading and display

[RandomFractals]

should be similar to #12 for user-defined snippets and #46 for project scoped snippets, but loading .code-snippets files instead.

Might require actually loading those snippets and checking scope for a language list to update top level languages in the snippets tree view.

see: https://code.visualstudio.com/docs/editor/userdefinedsnippets#_language-snippet-scope

[mig8447]

This would be nice, the tree would need to examine the scope on each snipped of the global files which may be time consuming but a cache could be built around it so that is not as expensive as time goes by