fix(toast): adding alert role for toast

[ChronosSF]

Closes #479

[ChronosSF]

@rkaraivanov , @desig9stein , So I played with this some more based on this document: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/alert_role

• Without role: alert the message is not read
• With role: alert NVDA reads it perfectly on show. As the docs state, changing the display property of a role: alert element is supposed to trigger a read from assistive software and that seems to be the case with nvda
• With the Windows Narrator, this works half of the time (basically each other toggle). I also tried using the "JS method" as described in the document (basically adding the role attribute on show and removing it on hide) but it didn't produce better results in Windows Narrator

@desig9stein could you please test role: alert on a Mac and see if it works?

[rkaraivanov]

You should move this to the connectedCallback hook.
As per the specification:

The element must not gain any attributes or children, as this violates the expectations of consumers who use the createElement or createElementNS methods.

[desig9stein]

@ChronosSF The reader on mac does not read anything with just the role, however, adding aria-live="polite" seems to fix the problem.

We use the same approach in Angular.

igx toast
aria-live

[ChronosSF]

Sadly, providing any aria-live seems to completely kill off the read out from Windows Narrator, but that seems to be the case with angular too. I guess having it read on Mac is better as we kinda go in the way of using nvda as baseline.