Move meta from this to external/contextual getter

[fregante]

Information about the sender/trace and options is currently stored on this. This is because:

• Most methods don't need access to this information
• It can be easily be left out of the method definition when not used
• It can be easily used regardless of parameter length
• It allows the the registration of any function, not just functions with specific parameters (e.g. arguments[0] is the meta")

this is fine for me, but maybe there alternative solutions. This is what I came up with:

import {getCurrentMeta} from "webext-messenger";

function whoAmI() {
	const meta = getCurrentMeta();
	return meta.trace[0];
}

This pattern:

• Preserves the previous wins
• Actually simplifies our types further (due to the lack of this)
• Somewhat matches other patterns like:

   import.meta.url; // Only available in the current file, in ES Modules
  
   function onMessageHandler() {
   	if (chrome.runtime.lastError) { // Available only in this call
   		
   	}
   }

The implementation would look like:

- await handler.apply(meta, args)
+ currentMeta = meta;
+ await handler(...args)
+ currentMeta = undefined;

and

export function getCurrentMeta() {
	return currentMeta;
}

[fregante]

Nope, I just realized that this could only work in sync functions, because a second message could replace currentMeta before first call returns. Even if I were to store each meta for each call, there's no way to match a meta with a call without passing a parameter… which would probably be this anyway:

const uid = getUID();
meta.set(uid, meta);
await handler.apply(uid, args)
meta.delete(uid);

and

function whoAmI() {
	const meta = getCurrentMeta(this);
	return meta.trace[0];
}

Which would be a marginal improvement over the current situation.