CSS Custom Highlights AX: Add support for IA2

Expose CSS Custom Highlights with type 'highlight' to IA2 with the text
attribute "mark:true". With this change, they're announced as "marked"
by screen reader NVDA.

Add tests to make sure the different types of highlights are announced
correctly.

In Firefox, NVDA also announces custom highlights with 'highlight' type
as "marked", JAWS doesn't announce them. Both announce the ones with
type spelling/grammar-error as spelling/grammar errors respectively.
Also tested this change with JAWS version 2025.2508.120 and the
'highlight' custom highlights are still not announced. NVDA used was
version 2025.3.0.52596.

Specifications of CSS Custom Highlight types:
https://drafts.csswg.org/css-highlight-api-1/#highlight-types

IA2 mark text attribute description:
https://github.com/LinuxA11y/IAccessible2/blob/master/spec/textattributes.md

Bug: 434641512
Change-Id: I1aa0ea0562be8888ec332d38b335466dfba7b871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6996629
Reviewed-by: Jacques Newman <janewman@microsoft.com>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Auto-Submit: Fernando Fiori <ffiori@microsoft.com>
Commit-Queue: Jacques Newman <janewman@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1522637}