Minify compile time constants during build

[ExE-Boss]

Previously SEND_RESPONSE_DEPRECATION_WARNING would end up as:

  const SEND_RESPONSE_DEPRECATION_WARNING = `
      Returning a Promise is the preferred way to send a reply from an
      onMessage/onMessageExternal listener, as the sendResponse will be
      removed from the specs (See
      https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/runtime/onMessage)
    `.replace(/\s+/g, " ").trim();

in both the built and minified distribution files, which adds unnecessary start-up overhead where the /\s+/g regular expression has to be compiled and executed, followed by trimming the string.

This instead performs such a process during the build, which provides a few milliseconds of start-up time savings per extension which uses this polyfill, which adds up for a lot of extensions, especially ones with an event page, where the polyfill has to be loaded and executed every time the extension handles an event.

[rpl]

I would prefer to don't do this, it looks like a bit of a micro-optimization but, if we really want to spare those milliseconds (e.g. if we have some kind of proof that for some extension those milliseconds are really going to sum up and become an actual performance issue), I would prefer to just rewrite that const to become a single-line string right into the polyfill sources (without applying these changes at build time).