For #74846

Migrates from using the heap service to have an explicit lifecycle

Follow up on #70354

Fixes #70354

This will break if getModel returns `undefined`

Readonly helps with type guards and code comprehension

This reverts commit 7b375fcb.

Reapplies 4a38520d with a fix for #66202

Fixes #66202

This reverts commit 4a38520d.

For #33413

Adds the active `SignatureHelp` object (along with the active signature) to the `SignatureHelpContext`.

Use modes.SignatureHelp as source of truth for activeSignature

Use the model to track state and active parameter instead of tracking this in the widget itself. This enforces a cleaner boundary between model and view

This allows us to store additional data on the state, such as the active parameter

The parameter hints model can only be in a single state at a time. Use a single state variable to better track this