Implement web page -> background script communication, with Firefox polyfill

[fregante]

• Follows Implement content script - unsafe script communication #5
• More context in Tighten/standardize unsafe context messaging (also useful to polyfill externally_connectible in Firefox) pixiebrix-extension#1015
• More context in Tighten externalProtocol messaging pixiebrix-extension#1019

[fregante]

A few notes regarding the unsafe communication in general:

• This API will need to accept an extension ID (for externally_connectable), which could be defined in getMethod("GET_VERSION", id) (since it's static) or at the call site, so getVersion(id)
• The unsafe context should probably not have access to arbitrary tabs or contexts, so getVersion({extensionId, tabId, frameId}) should not be supported
• Since this is now being built as a public module, the unsafe messaging should probably always indicate which extension it's trying to talk to, even if the content script doesn't require the extension ID.

This leaves only 3 contexts accessible from the unsafe context:

• current CS
• background context (either via externally_connectable or via CS)
• perhaps the current tab’s editor/devtools (either directly or via CS+background)

Since the unsafe context is special, security-wise, it should probably have its own special methods for each context anyway.

General notes also added in #5

[twschiller]

I agree with everything above. As you mention, the external -> devtools isn't strictly necessary because it can hop through the background page. (I don't think CS can initiate communication to the devtools in our current setup?)

Also commented in #5 (comment) about opt-in for endpoints