Add a configurable partial approval emoji

[jp-ayyappan]

What does this PR do?

Adds an optional emoji-partially-approved action input. When at least one approval exists but the configured approval threshold has not been reached, Slapr uses that reaction. Once the threshold is met, Slapr replaces it with the configured approved reaction. Changes requested continue to take precedence.

Motivation

Slapr already supports a configurable approval count, but it does not expose progress toward that count. This lets a workflow use ⏭️ while approvals are still pending and 🚢 when the threshold is met.

Describe how you validated your changes

• Added tests for partial approval, threshold completion, backward compatibility, comment precedence, and changes-requested precedence.
• All 32 unit tests pass locally.
• The Docker image builds successfully and passes the repository import smoke test.

Additional Notes

The new input is opt-in and has no default, so existing workflows retain their current behavior. Broader approval-policy configuration is intentionally left for focused follow-up changes.