[IDE] Improve presentation of included context source references

Problem to solve

The way we display context sources can be improved by removing the unnecessary included references collapsible and supporting token focus using arrow keys.

See gitlab-org/editor-extensions/meta#223 for more details on the problem and user feedback.

Why it's important

This will make it easier for users to format prompt instructions using only the keyboard as well as better aligning with the North Star designs.

Proposal

Scenario: Context sources have been added to the current prompt
  Given the a context source has been added to the current prompt
  Then the tokens are presented inside the prompt input
  And the user can select a token using the arrow keys 
  And the user can remove focused tokens by pressing backspace

Example

Current	Proposed

Important links

• Figma
• Prototype iteration 3 (to illustrate the concept, not a 1:1 spec)