Meta: Add LibUnicode and LibLocale to gn build

A lot of code gen happening here. These generators are kind of
awkward to work with, and the fact that the CLDR data download
extracts over 8,000 files makes it hard to fit into the explicit
patterns GN expects of us.

Meta/gn/secondary/Userland/Libraries/LibUnicode/enable_unicode_download.gni

@@ -0,0 +1,7 @@
+declare_args() {
+  # If true, download CLDR and UCD files from unicode.org or the unicode
+  # consortium's github and use it in LibUnicode and LibLocale.
+  # Data will be downloaded to $cache_path/CLDR, $cache_path/UCD
+  # and $cache_path/EMOJI
+  enable_unicode_database_download = true
+}