build: add husky ignored files to .gitignore (angular#39388)

Ahead of upgrading to husky v5, adding the shell file location
to .gitignore to prevent it from randomly showing up when devs
checkout older branches.  Beginning in v5, husky places its
shell files in a directory at `.husky/_` so these are placed
in the .gitignore to prevent being commited or tracked.

PR Close angular#39388

.gitignore

@@ -51,3 +51,6 @@ baseline.json
 
 # CLDR data
 tools/gulp-tasks/cldr/cldr-data/
+
+# Husky
+.husky/_