Receive `tabId`-only messages on runtime pages

[fregante]

Reported in https://github.com/pixiebrix/pixiebrix-extension/pull/4208#discussion_r962044715

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Currently, the only way to message chrome-extension:// pages is by specifying a page, with an optional tabId. If a page is missing, it's assumes that a message is supposed to be forwarded:

webext-messenger/source/thisTarget.ts

Lines 96 to 98 in 7d66d2e

	if (!to.page) {
	return "forward";
	}

Improvement

• Don't assume that a page-less target must be forwarded

Related

• Messages are forwarded twice if 2 runtime pages are open #85

[fregante]

As part of this, I should probably also start testing the options.html page that already exists in the test extension.