refactor: auto generate electron_version.h from the version file (#18866)

* refactor: auto generate electron_version.h from the version file

* Update BUILD.gn

Co-Authored-By: Jeremy Apthorp <nornagon@nornagon.net>

build/generate-template.py

@@ -0,0 +1,16 @@
+import json
+import sys
+from string import Template
+
+inpath = sys.argv[1]
+outpath = sys.argv[2]
+argpaths = sys.argv[3:]
+
+with open(inpath, 'r') as infile, open(outpath, 'w') as outfile:
+  data = {}
+  for argpath in argpaths:
+    with open(argpath, 'r') as argfile:
+      data.update(json.load(argfile))
+
+  s =  Template(infile.read()).substitute(data)
+  outfile.write(s)