[API] Add type to 'OutliningSpan' class

[mjbvz]

From microsoft/vscode#20802

Search Terms:

• code folding
• outliningspans

Problem
Allow users to fold / hide blocks of imports such as :

import {} from 'a';
import {} from 'b';
import {} from 'c';

Proposal
Add import blocks to the outliningSpans introduced in #22400.

We should also consider marking import block spans in some way. This could be something like:

interface OutliningSpan {
    ...

    type: 'import' | 'code'
}

Here's the vscode proposed folding API: https://github.com/Microsoft/vscode/blob/0c8e31f14a2c1d4c0c7ea75573caab39b171384b/src/vs/vscode.proposed.d.ts#L10

/cc @aeschli

[mjbvz]

Making this issue more generic to track adding a type to the OutlingSpans object. Proposed set of OutliningSpans types:

• 'import'
• 'comment' - TSServer: Way to identify a folding range as being for a comment  #22932
• 'region' - [typescript] add type to #region fold ranges vscode#44530
• 'code'