Describe the bug
The new Block Inserter doesn't constrain tabbing with the Tab key.

In the old one, tabbing was constrained. Although we're not treating the Block Inserter as a modal, I think the previous behavior was beneficial. Not sure about the reasoning behind the removal of the tabbing constraining. I'd appreciate anyone pointing me to the discussion where this behavior removal was considered.

To reproduce

• test on default WordPress 5.4 block editor first
• open the main block inserter:

• focus is moved within the inserter
• use the Tab key to navigate the inserter content
• see that tabbing is constrained within the inserter: once you reach the last focusable element, focus is moved to the beginning and vice-versa
• activate the Gutenberg plugin, right now it's version 8.2.1
• open the main block inserter: you get the new one:

• use the Tab key to navigate the inserter content
• by default the inserter displays 70 blocks so you will need to press Tab 70 times
• I'll refrain from expressing usability concerns about the removal of the expandable block groups: having to tab dozens of times, or scroll through a long list of blocks, doesn't seem a great improvement to me
• once you reach the last focusable element, tabbing is not constrained within the inserter any longer (and vice-versa)
• keeping tabbing exits the inserter and moves focus to the rest of the UI, which isn't ideal

Expected behavior
Tabbing to be constrained.

Editor version (please complete the following information):

• WordPress version: 5.4
• Gutenberg plugin 8.2.1