Copied command_exists function to the helpers lib so that there is no hidden dependency on the base plugin

The original command_exists function will stay in the base plugin, but will no longer be used by other plugins or themes.

test/lib/helpers.bats

@@ -30,6 +30,26 @@ function local_setup {
 # TODO Create global __get_base_name function
 # TODO Create global __get_enabled_name function
 
+@test "helpers: _command_exists function exists" {
+  type -a _command_exists &> /dev/null
+  assert_success
+}
+
+@test "helpers: _command_exists function positive test ls" {
+  run _command_exists ls
+  assert_success
+}
+
+@test "helpers: _command_exists function positive test bash-it" {
+  run _command_exists bash-it
+  assert_success
+}
+
+@test "helpers: _command_exists function negative test" {
+  run _command_exists __addfkds_dfdsjdf
+  assert_failure
+}
+
 @test "helpers: bash-it help aliases ag" {
   run bash-it help aliases "ag"
   assert_line "0" "ag='ag --smart-case --pager=\"less -MIRFX'"