hide admin menu separators for screen readers

[afercia]

When a screen reader reads out the Admin menu, it may announce (depending on the user settings) how many items are inside the menu list. For example, the default Admin menu has 11 first level items and NVDA will announce:

"navigation landmark list with 13 items"

NVDA says "13" because it takes into account all the LI elements and 2 of them are just visual separators: empty LI with no link or text.

Screen reader users may be confused because they're informed about "13" items but then they can navigate to just 11 of them (about the last item, the "Collapse menu", see #29958).

Proposed patch just adds an aria-hidden=true attribute to separators, you can see the result in the NVDA Speech Viewer window in the screenshot below.

@screen reader users, the default first level 11 items are:

• Dashboard
• Posts
• Media
• Pages
• Comments
• Appearance
• Plugins
• Users
• Tools
• Settings
• Collapse menu

[joedolson]

This looks like a good solution; recommend doing this, definitely.

[helen]

I didn't realize that's how we detect separators, ugh. Patch is fine though, will drop it in.

[helen]

In 30393:

Accessibility: hide admin menu separators from screen readers.

props afercia. fixes #30010.