Omit natively-supported filetypes from packaging

This is one of the more drastic changes to the build process, but it
seems to be a good one. Having two copies of the same filetype can cause
a variety of problems since Vim tries to load both of them - for
example, :setf ada will error if you have mainline vim-polyglot
installed, because the Ada autoload file uses :lockvar. Not copying the
filetype support that's bundled with Vim already prevents that collision
issue.

This change also means you can immediately get the latest version of the
natively-supported filetypes if you keep your Vim up to date, while the
version bundled into vim-polyglot is still pinned at v8.2.4274 for
compatibility reasons. (I did try bumping up that version number, but
you end up getting Vim9 script, which will break vim-polyglot on older
Vims.)

scripts/build

@@ -496,7 +496,7 @@ def extract(packages)
   output = []
   packages.map do |package|
     copied_something = false
-    if !package["remote"]
+    if package["native"] or !package["remote"]
       next
     end
     repo, branch, path, dir = parse_remote(package["remote"])