Destinations as first-class objects

[pokey]

The problem

Today, our handling of positions is inconsistent. This problem manifests itself in multiple ways:

• We allow "end of" to appear in places that expect a position target, which means that "paste end of air" is a valid command, making it nearly impossible to get Talon to hear "take air past end of item bat". It instead hears two commands: "take air" then "paste end of item bat"
• We need to have the edit action on regular targets, and position targets need all the methods from regular targets that they don't actually support
• Targets have no strongly typed way of indicating that they expect a position
• Position targets are hacked to set delimiter to "" when position is "end of" or "start of"

The solution

We introduce a new type called a Destination, both on the Talon and extension side. This type is distinct from a Target, and will be used everywhere we expect a PositionTarget today (eg "paste", second target of "bring", etc).

Destination will be have the following interface:

type InsertionMode = "before" | "after" | "to";

interface Destination {
  insertionMode: InsertionMode;
  target: Target;
}

It will exist in the following places:

• As a capture Talon side
• As a new descriptor type, similar to today's TargetDescriptor
• As a new post-processTargets type, like today's Target

It will involve the following steps:

• Rework Talon side to use destinations #1440
• Implement extension side of destinations #1630