support all merge methods

[robandpdx]

This pull request primarily focuses on enhancing the merge functionality in the app.js file and updating the tests in the test.js file to accommodate these changes. The key changes include the addition of a dynamic merge method selection based on the repository's settings and the corresponding updates in the test cases to validate these changes.

Merge method enhancement:

• app.js: Added a dynamic merge method selection logic. Based on the repository's settings, it now chooses between 'merge', 'squash', or 'rebase' as the merge method. Previously, it defaulted to use 'merge' as the merge method.

Test case updates:

• test.js: Updated the test payload to include allow_merge_commit, allow_squash_merge, and allow_rebase_merge properties to mimic the repository's settings. This was done in the payloadPrLabeled and payloadPrLabeledByBot objects. [1] [2]
• test.js: Updated the after.each test cleanup method to reset the merge settings.
• test.js: Updated and added new test cases to validate the dynamic merge method selection logic. These tests ensure that the correct merge method ('merge', 'squash', or 'rebase') is used based on the repository's settings.

This closes #13

[github-actions]

=============================== Coverage summary ===============================
Statements   : 100% ( 150/150 )
Branches     : 100% ( 88/88 )
Functions    : 100% ( 27/27 )
Lines        : 100% ( 149/149 )
================================================================================