Sync .gitignore and extensions.json across repos (#367)

Author: sdwheeler

.gitignore

@@ -1,10 +1,22 @@
-log/
-obj/
 _site/
-.optemp/
-_themes*/
-_repo.*/
-
+_themes/
+.DS_Store
+.openpublishing.build.mdproj
 .openpublishing.buildcore.ps1
-.vscode/styles
+.optemp/
 .vale/
+.vscode/launch.json
+.vscode/styles/
+*.orig
+**/.ignore/**
+**/settings.json
+dependentPackages/
+log/
+maml/
+obj/
+packages.config
+packages/
+StaleContentReport.*.csv
+Tools/NuGet/
+updatablehelp/
+xhtml/