Use pure dependency injection

[pokey]

Context

• We currently use a custom built dependency graph for dependency injection, which is effectively a service locator (anti-)pattern
• We have recently started adding global singletons, most notably our ide()
• These global singletons result in subtle state problems whereby the test harness and Cursorless VSCode extension end up with different copies of the singleton
• We are introducing new scope handlers that need access to more components from the graph

Options considered

• A DI library (probably tsyringe), used as a service locator like our existing graph
• A DI library used in a composition root
• Pure DI used in a composition root

Decision

• We have decided to use Pure DI
• This entails migrating all of our graph components to constructor arguments
• Then a composition root close to our extension.ts will inject all dependencies

Consequences

• No magic to learn. This might not be that big of a deal because the contributor base isn't super large, but we tend to have contributors that only contribute one or two PRs, so quick learning curve is important
• Potentially large composition root; we will see
• If we have tightly coupled components that instantiate one another, we may end up drilling their dependencies down through many constructors, but we will try to avoid this by keeping hierarchy flat / using loose coupling
• Composition root could become point of contention
• Frequent changes to constructor signatures?
• Dependencies of components very clear, ie no hidden dependencies on ide()
• It is a bit more cumbersome to add a new dependency to a component; need to add constructor arg and modify composition root