Bug 1933293 - Rework aboutWelcome modal behavior to still show about:welcome page and make requiring action to proceed configurable r=firefox-desktop-core-reviewers ,mossop,pdahiya

This patch updates the logic that prevents about:welcome from showing when the Nimbus `aboutWelcome` `showModal` variable is set to true. If separate screens for the modal are provided, it can now be shown on top of the default onboarding flow in about:welcome.

A `requireAction` variable that sets the modal to be a window modal requiring action (other than pressing ESC) to dismiss is also added to the `aboutWelcome` Nimbus feature

Differential Revision: https://phabricator.services.mozilla.com/D230185

browser/components/BrowserContentHandler.sys.mjs

@@ -817,7 +817,12 @@ nsBrowserContentHandler.prototype = {
           case OVERRIDE_NEW_PROFILE:
             // New profile.
             gFirstRunProfile = true;
-            if (lazy.NimbusFeatures.aboutwelcome.getVariable("showModal")) {
+            // If we're showing the main onboarding content in a modal, skip
+            // showing about:welcome as the homepage.
+            if (
+              lazy.NimbusFeatures.aboutwelcome.getVariable("showModal") &&
+              !lazy.NimbusFeatures.aboutwelcome.getVariable("modalScreens")
+            ) {
               break;
             }
             overridePage = Services.urlFormatter.formatURLPref(