build: add husky ignored files to .gitignore (#20856)

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.

Author: josephperrott

.gitignore

@@ -41,3 +41,4 @@ testem.log
 /.bazelrc.user
 *.log
 .ng-dev.user*
+.husky/_