Draft: Add support for building base-browser

Closes #40522 (closed), #40501 (closed), #40500 (closed), #40499 (closed), #23656 (closed).

I was able to build a base-browser with this additional patch (since we don't have a base-browser branch in tor-browser.git yet):

From b79eb6d759c768dc59e6c2e6cf714eb69b470ecd Mon Sep 17 00:00:00 2001
From: Nicolas Vigier <boklm@torproject.org>
Date: Wed, 22 Jun 2022 12:33:35 +0200
Subject: [PATCH] (not for merge) set firefox git_hash for base-browser, for
 testing

---
 projects/firefox/config | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/projects/firefox/config b/projects/firefox/config
index 51ce8b87..a52f127e 100644
--- a/projects/firefox/config
+++ b/projects/firefox/config
@@ -58,12 +58,16 @@ steps:
         use_container: 0
 
 targets:
+  basebrowser:
+    git_hash: f8dfbe774b80e4545978e7ee69d97ec29294f761
+    tag_gpg_id: 0
+
   release:
     var:
       branding_directory: 'browser/branding/official'
 
   nightly:
-    git_hash: '[% c("var/project-name") %]-[% c("var/firefox_version") %]-[% c("var/browser_branch") %]-1'
+    git_hash: '[% IF c("var/base-browser"); GET "f8dfbe774b80e4545978e7ee69d97ec29294f761"; ELSE; c("var/project-name") %]-[% c("var/firefox_version") %]-[% c("var/browser_branch") %]-1[% END %]'
     tag_gpg_id: 0
     var:
       branding_directory: 'browser/branding/nightly'

I've launched this base-browser on Linux, and it opens a browser that looks like firefox.