[KTextbox] Add slots to append elements before or after input

[MisRob]

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Sub-issue of learningequality/studio#5060.

Complexity: Medium

Summary

Add 4 new slots to KTextbox via which it will be possible to append elements before or after input, outside as well as inside:

• innerBefore
• innerAfter
• outerBefore
• outerAfter

When content placed to inner slots, ensure that typing cursor starts after / ends before the slot area (= input text never overlaps slot areas).

These slots will be used for use-cases such as:

Guidance

• Getting started
• How to update library
• How to update the documentation website
• Visual testing guide
• Writing guidelines

Acceptance criteria

• Is LTR and RTL compliant
• KTextbox documentation page has live examples demonstrating use of the slots
• There are visual tests

[MisRob]

As per learningequality/studio#5303 (comment), reserved for @Abhishek-Punhani if you'd still like to work on this.

[Abhishek-Punhani]

As per learningequality/studio#5303 (comment), reserved for @Abhishek-Punhani if you'd still like to work on this.

Yes @MisRob , I would love to work on this.