Add button to namespace nodes to create category folders

[isc-bsaviano]

This PR fixes intersystems-community/vscode-objectscript#1161. I have three questions that I'd like to resolve before merging:

1. Should I change the name of the command? This is the best I could come up with without referencing Studio.
2. Should the icon change when Alt is pressed? I couldn't find a good one to express "tree, but readonly".
3. I think that five inline icons is too many, and I'd like to remove the SMP link one. Thoughts?

[isc-rsingh]

IMO using "Studio style" or something like that in a command name is fine if it's mainly for devs coming from Studio, and it's the most concise way to describe the functionality.

[isc-bsaviano]

I would prefer to not reference Studio unless it's a feature that's directly related to migration, like importing server definitions or snippets. I will request that documentation for this feature be added to the "Migrating from Studio" page so Studio users will know what it is.

[gjsjohnmurray]

1. I think "Categories" is OK, given that you don't want "Studio-style" like my extension uses.
2. I think list-unordered would be tolerable, and I'd rather the icon changed in response to the Alt key (as the others with that capability do) than this one didn't and its changed tooltip was the only hint of the alternative mode.
3. We don't know how many people use the SMP button, so I'd rather not drop it. Arguably it's of use to more people than the new Categories button. As you know, I like that my existing extension can simply be disabled by folk who don't need its button, but I didn't convince you not to embed the feature into Server Manager rather than keeping it standalone and adding it to the extension pack.