[IDE] Disable Duo's send button when the prompt input is empty

Problem to solve

Showing the button as enabled when there's no ability to send prompt instructions gives a false impression of the system's current state.

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

Why it's important

Fixing this adds UI polish and helps draw attention to the submit button only when it's relevant.

Proposal

1. Only enable the button when the prompt input has a value
2. Switch the send button to ↵

Current	Proposed

Important links

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