Material-components-web-components: mwc-menu closes on selection

[motiejunas]

Describe the bug
When using mwc-menu together with mwc-check-list-item any new selection closes the mwc-menu.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://material-components.github.io/material-components-web-components/demos/menu/
2. Click on OPEN GROUPED MENU
3. Imagine that this is a multi select menu (didn't find better demo there)
4. Select any option
5. mwc-menu gets closed

Expected behavior
mwc-menu stays open.

Additional context
When using mwc-menu together with multi attribute my expectation would be that the menu remains open after a selection or, even better, some additional attribute such autoClose would work too.

[asyncLiz]

We spoke with design, and we agree that this is a valid UX pattern. This needs to be upstreamed into the MDC foundation as an option to prevent the menu from closing.

Unlike our demo, which uses a list item with a checkmark icon, design recommends using a <mwc-check-list-item> with the checkbox graphic for a persistent multi-select list that would remain open until the user clicked outside the bounds of the menu.

[yinonov]

following the code leads to the conclusion this is it's blocked by the lack of relevant setMulti method on mdc's select foundation
Why is material-components/material-components-web#6033 close then?

Seem like mwc's list depends on mdc's list foundation as well
https://github.com/material-components/material-components-web-components/blob/d10fda0a0b663f1e0be185ac2735123ce227a33a/packages/list/mwc-list-base.ts#L66