a11y: make ins/del diff markup accessible to screen readers

[hiteshdugar]

Problem

When HTML diff output is displayed in richTextFieldWithTables in readOnly mode, added text (green) and removed text (red strikethrough) are conveyed through visual styling only. Screen readers read all text as plain content with no indication of what was added or removed — violating WCAG 2.1 SC 1.3.1.

Changes

index.js

1. added "ins", "del" to ALLOWED_TAGS;
2. added makeInsDelAccessible() and escapeAttr() functions; called makeInsDelAccessible() in readOnly path of setEditorContents()

custom.css

1. added ins { text-decoration: none; } and del { text-decoration: none; }

appian-component-plugin.xml

1. version 1.19.0 to 1.19.1

Backward compatible. No impact on editing, pasting, saving, or existing readOnly content.