Provide suggestion for treqs-links only when within a treqs-element

User Story

As a developer, I want to use autocompletion when linking requirements, so that I don’t need to handle IDs.

Traceability

• Related to: #
• T-Reqs ID:
• Requirement label:

Acceptance criteria

• Suggestion for treqs-link is offered only when text cursor is within a treqs-element tag