Added the Windows installer files (Base Browser).

Added the strings for the Windows installer. See tpo/applications/tor-browser-build!976 (merged).

If the format I chose is okay, we can wait for creating a Weblate component, but having the source file here would allow to get the localization machinery started.

en-US/windows-installer/strings.ini

+[strings]
+; & denotes the shortcut; please use && if you need to insert a single &.
+add_shortcuts=&Add Start menu and desktop icons
+; %(program)s is, for example, "Tor Browser"; %(version)s is a Windows version (e.g., 7, 10).
+min_windows_version=%(program)s requires Windows %(version)s or later.
+destination_exists=The destination folder already exists. Do you want to continue anyway?