Add theme function to process contextual trigger text.

Problem/Motivation

In 📌 Contextual Links Style Update Needs work , we've run into some circumstances where it is beneficial to perform some formatting on screenreader-only trigger text.

Steps to reproduce

Proposed resolution

• Add a theme function that simply returns the contextual trigger text that it is provided as an argument. This will result in no changes to how a user experiences contextual links, but it will provide themes the ability to modify the contents of the trigger text.
• Contextual Links AuralView JavaScript should process the trigger text with this theme function.

Remaining tasks

If 📌 Contextual Links Style Update Needs work has landed, there may be a @todo referencing this issue.

User interface changes

API changes

Data model changes

Release notes snippet