feat(block): prevent disabled block content from being tabbed into

[jcfranco]

Related Issue: #548

Summary

This sets tabIndex on the host component to prevent shadow DOM content from being accessed via the keyboard.

From WICG/inert#48:

From step 3 of https://w3c.github.io/webcomponents/spec/shadow/#sequential-focus-navigation

[...] an element whose tabindex value is negative must not be appended to NAVIGATION-ORDER.

Which in other words means that if an element has tabindex=-1 and a shadowRoot, the focus won't be able to reach the element's contents (distributed or not) 🎉

A downside to this approach is that the user-set tabindex is not preserved when a block is disabled and reenabled (or vice versa).

Another change in this PR is that any slotted control is no longer hidden when the block is disabled since the component is no longer interactive.

cc @kat10140

Questions

• Are we OK preventing user select on Scrim?
• Should this be considered a breaking change? Reasoning: whole block is no longer interactive when disabled -> Yes

[driskull]

👍 nice