WIP: Add support for named targets (like the sidebar)

[fregante]

• Part of How can a content script target a frame? #41

This enables:

// action.ts
registerTarget("sidebar")

// contentScript.ts
doSomething({name: "sidebar"}, "foo"); // Automatically uses the current tabId

// Anywhere
doSomething({name: "sidebar", tabId: 123}, "foo"); // Can be used from anywhere

With a simple change this will also allow name-less/tab-less calls to target the content script from the sidebar or other frames:

// action.ts
doSomething({}, "foo"); // Automatically uses the current tabId

---

tabId-less messages will always go through the background page, unfortunately, where the target is figured out either via "named targets" map or by using TOP_FRAME_ID on the sender’s tabId.

To do

• Add tests
• Cleanup
• Close WIP: Add support for direct iframe messaging #42 if it replaces it fully

[fregante]

Same as pixiebrix/pixiebrix-extension#1695 (comment)

90% of these changes are now in #49

The remaining 10% were undone in this commit: 9c71b72 (#49)