Add activeSignatureHelp to SignatureHelpContext

[mjbvz]

For #33413

Adds the active SignatureHelp object (which has the correct activeSignature set) to the SignatureHelpContext. This allows signature help providers to use the activeSignature info to take into account if the user has arrowed through signature help results

Todo:

• Tests are failing. Not part of this change directly but a logic error that this change revealed
• How can we ensure that custom SignatureHelp objects are not lost when returned through the SignatureHelpContext?

[jrieken]

How can we ensure that custom SignatureHelp objects

We have two ways to do this:

• Implement a custom lifecycle and custom caching, e.g. as done for IntelliSense
• Use the heap service. That's more convenient but less performant since the main thread walks all objects. This is how it works: On the ext host use ExtHostHeapService#keep which will keep the object and mix-in an id, on the main side use IHeapService#trackRecursive. Once the main side object is GC'ed, the ext-side object will be removed

[mjbvz]

Merging in to proposed api