side-bar icon margin on RTL

### Issue type

* [X] bug report
* [ ] feature request

### Issue description

icons on rtl mode get a left margin and the text (that align from the left side of the icon)
is without any space from the icon

**Expected behavior:**
text and icon should be separated by margin / padding.

**Steps to reproduce:**
just add menu item in rtl mode
and you can see it live on ngx-admin template on rtl mode
https://www.akveo.com/ngx-admin/pages/dashboard

im on nebular ^4.5.0

a fix for that should be (on rtl):
```
nb-menu .menu-item .menu-icon {
margin: 0 0 0 0.5rem;
}
```