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.

(cherry picked from commit 086002f)

Author: josephperrott

.gitignore

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