feat(menu): add md-menu standalone component

[kara]

This is a small PR to add styling for the basic menu component.

TODO:

• menu toggle that will show/hide the menu
• keyboard navigation
• aria attributes

[googlebot]

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[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request) and all commit authors, but as best as we can tell these commits were authored by someone else. If that's the case, please add them to this pull request and have them confirm that they're okay with these commits being contributed to Google. If we're mistaken and you did author these commits, just reply here to confirm.

[jelbourn]

I kind of want to stop putting stuff into mixin.scss since it's too much of a random grab-bag. Can we add a button-mixins file?

[kara]

Yes, good call. Added.

[jelbourn]

Not sure what's up with the CI checks

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[kara]

@jelbourn Comments should be addressed. The CLA thing is weird but it seems like it resolved itself?

[jelbourn]

LGTM

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