feat: custom branch name templates (#571)

* Add branch-name-template config option

* Logging

* Use branch name template

* Add label to template variables

* Add description template variable

* More concise description for branch_name_template

* Remove more granular time template variables

* Only fetch first label

* Add check for empty template-generated name

* Clean up comments, docstrings

* Merge createBranchTemplateVariables into generateBranchName

* Still replace undefined values

* Fall back to default on duplicate branch

* Parameterize description wordcount

* Remove some over-explanatory comments

* NUM_DESCRIPTION_WORDS: 3 -> 5

test/create-prompt.test.ts

@@ -61,6 +61,7 @@ describe("generatePrompt", () => {
       body: "This is a test PR",
       author: { login: "testuser" },
       state: "OPEN",
+      labels: { nodes: [] },
       createdAt: "2023-01-01T00:00:00Z",
       additions: 15,
       deletions: 5,
@@ -475,6 +476,7 @@ describe("generatePrompt", () => {
         body: "The login form is not working",
         author: { login: "testuser" },
         state: "OPEN",
+        labels: { nodes: [] },
         createdAt: "2023-01-01T00:00:00Z",
         comments: {
           nodes: [],