filetype: Add a module to wrap vim.filetype.add (#277)

tests/test-sources/modules/filetypes.nix

@@ -0,0 +1,41 @@
+{
+  example = {
+    filetype = {
+      extension = {
+        foo = "fooscript";
+        bar.__raw = ''
+          function(path, bufnr)
+            if some_condition() then
+              return 'barscript', function(bufnr)
+                -- Set a buffer variable
+                vim.b[bufnr].barscript_version = 2
+              end
+            end
+            return 'bar'
+          end
+        '';
+      };
+      filename = {
+        ".foorc" = "toml";
+        "/etc/foo/config" = "toml";
+      };
+      pattern = {
+        ".*/etc/foo.*" = "fooscript";
+        ".*/etc/foo.*%.conf" = [
+          "dosini"
+          {priority = 10;}
+        ];
+        "\${XDG_CONFIG_HOME}/foo/git" = "git";
+        "README.(a+)$".__raw = ''
+          function(path, bufnr, ext)
+            if ext == 'md' then
+              return 'markdown'
+            elseif ext == 'rst' then
+              return 'rst'
+            end
+          end
+        '';
+      };
+    };
+  };
+}