replace 'iso-8859-2' with plain JS

[sosnovsky]

It seems we use iso-8859-2 only at

flowcrypt-browser/extension/js/common/core/mime.ts

Line 485 in 71b033b

return Iso88592.decode(node.rawContent!);

But now this functionality can be implemented with plain js, using TextDecoder, like this:

const decoder = new TextDecoder('iso-8859-2');
return decoder.decode(node.rawContent!);