Display registered settings groups by extension

[roblourens]

The "configuration" extension contribution point can take a single object of setting keys -> definitions or an array of these, each with their own title. Currently the Extensions node in the settings editor TOC shows extension settings by config section with the registered title, mixed up globally and sorted alphabetically. A couple issues with this:

• Config section titles can be generic and duplicate other sections
  
• There is a LESS section that comes from the builtin css-language-features extension, but there is no way to know which extension it came from
• If an extension registers multiple sections, they are not grouped together and the specified order is ignored. The split json editor handled this correctly.

So I'm thinking we should actually have the extension name as the top node, put the extension sections under that with their given titles and order. This actually makes extension sections much more useful. This is a good start towards giving extensions more control over the presentation of their settings, like discussed in #70589, and we don't have to extend JSON schema to do it.

[roblourens]

cc @eamodio, may be interesting to you

[eamodio]

I definitely think this makes a lot of sense -- keeping the extension settings contain with in its own "space"

[rzhao271]

WIP

Here's how a single level of grouping looks: