feat(menu): support typeahead focus

[crisbeto]

Adds support for focusing menu items via the character keys.

Note: I'm not sure whether using all top-level text nodes to determine the menu item text isn't too big of an assumption (e.g. it won't be accurate for the case where all the content is wrapped in a strong tag). Alternatively I can add a special case for md-icon or just take the textContent verbatim.

[julianobrasil]

@crisbeto, any chance to do something like this on mat-select too?

[crisbeto]

@julianobrasil this has been available on the select for a while now.

[jelbourn]

The W3C authoring practices don't mention this behavior for menus- does this come from a specific resource?

[crisbeto]

Not from a specific resource @jelbourn, but it's something I've noticed being done natively on MacOS. Windows also has something similar, although it executes the item action as well.

[jelbourn]

I sent off a question to our internal a11y folks to see if there's any reason to avoid this behavior.

[jelbourn]

Feedback is that it is good to add.

[jelbourn]

LGTM

[jelbourn]

Just needs rebase

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