feat(ui5-menu-separator): add new component

[didip1000]

New component that can be slotted with menu items

BREAKING CHANGE: startsSection property removed from MenuItems

Before:

<ui5-menu>
    <ui5-menu-item text="Item A"></ui5-menu-item>
    <ui5-menu-item text="Item B" starts-section></ui5-menu-item>
</ui5-menu>

Now:

<ui5-menu>
    <ui5-menu-item text="Item A"></ui5-menu-item>
    <ui5-menu-separator></ui5-menu-separator>
    <ui5-menu-item text="Item B"></ui5-menu-item>
</ui5-menu>

[Add image for how it looks once done]

!! PR & DESCRIPTION A WIP !!

[vladitasev]

The "Now:" section of the commit description still contains the removed property (starts-section)

[didip1000]

There is hover effect over the menu-separator in the menu. And we need to fix it. setting the pointer events to none might do the job.

Setting pointer events to none will cause the focus to go back to the parent, but I removed the hover effect with css

[didip1000]

Also the API of the component looks strange when all jsdoc related comments are addressed:

We reworked the separator and it no longer inherits from ListItem so those properties and elements are not being inherited either :)

[unazko]

Retested. The solution continues to work as expected.