Composable RCE lifecycle

[KTrain5169]

Currently, almost the entirety of the RCE lifecycle lives in one async function RCEActionHandler.

This was okay when the extension was smaller, but adding more features means that it becomes a bit harder to:

• Disable each component individually when debugging and testing
• Integration test each component
• Partial test an action's lifecycle

Also see #303, which this issue is a child of.

To fix the problems above and pave the way forward for #303, the following needs to be built:

• Separate functions for each lifecycle component (RCEActionHandler becomes just an assembly of these components)
• Separate "test handler" that mocks the "production assembly" (WIP name: RCEActionTester | RCEActionMocker)
• A way to switch on or off one or more components while in debug or testing mode (dev-only settings doesn't sound like a good idea, but it seems the most obvious for right now).