fix: solution merge PR timeouts and conflicts

[ewingjm]

Purpose

Resolves #65, resolves #64. Solution merge PRs were being frequently created with merge conflicts and solution merges were occasionally getting stuck on a 'Merging' status due to solution export timeouts.

Approach

Fixes the merge script to use async solution exports and create fresh branches from master even when a source branch is provided.

TODOs

• [x] Automated test coverage for new code
• [x] Documentation updated (if required)
• [x] Build and tests successful