Export `thisTarget` to support messaging a Chromium side panel

[twschiller]

Context

• I need to be able to message the Chromium side panel for a cross-context generator
• I can't use this on the message receiver because the side panel metadata doesn't appear in the trace
• I think exporting thisTarget would allow it?:

  webext-messenger/source/thisTarget.ts

  Line 43 in 7a50772

  export const thisTarget: KnownTarget = (() => {

• Code reference: https://github.com/pixiebrix/pixiebrix-source/blob/feature%2F2047-copilot-extension/apps/browser-extension/src/extensionPages/extensionPagePlatform.ts#L306-L306

I'm trying to send to the target to the background so that the background initiate messages for the generator back to the target:

[fregante]

Who is sending the first message? If it's the sidebar, then "this target" is already part of the trace, so it can be stored and reused later:

https://github.com/pixiebrix/pixiebrix-source/blob/e9dab31111a4b03c8f7131de2aa6fd093748405a/apps/webext/src/background/capture.ts#L209

What may be missing is a helper to turn that trace object into an usable target, which we kind of have here in the same file:

https://github.com/pixiebrix/pixiebrix-source/blob/e9dab31111a4b03c8f7131de2aa6fd093748405a/apps/webext/src/background/capture.ts#L80-L95