feat: implement branch creation script (#17)

This Pull request implements the branch creation script whose primary
function is to create a new branch from the forked repository for an
authenticated user.

### Changes Made 

- Implemented the main `createBranch` function in the `branch` script at
`src/lib/branch.js`; this function takes in 3 params;
- `userOctokit` - a user authenticated instance of octokit that can be
used to perform action on user's behalf
- `repoDetails` - the user's forked repo details; hold the
`repoFullname` and `repoMainBranchRef` as properties in the object type
param
- `newBranchName` - name of the branch that will be created for the user
  
The `createBranch` function performs the following operation in the
stated order...
- It initially fetches the forked repository main branch where we wished
to create the new branch off of; it does this using the `getBranch`
helper function; this return the `SHA` of the branch
- Then we make a request to the endpoint `"POST
/repos/{owner}/{repo}/git/refs"` to create the new branch; passing in
the `repoDetails.repoFullname` properties, the head branch `SHA` and the
`newBranchName`.

- Moved the `getBranch` function from the `fork` script to the `branch`
script

### Screencast/Screenshot


[screencast-bpconcjcammlapcogcnnelfmaeghhagj-2024.04.01-13_54_14.webm](https://github.com/babblebey/jargons.dev/assets/25631971/62a2f9b1-7643-42e3-b91f-1427fad1c170)

Author: babblebey

src/lib/branch.js

@@ -0,0 +1,50 @@
+import { getRepoParts } from "./utils/index.js";
+
+/**
+ * Create a new branch on a given repository
+ * @param {import("octokit").Octokit} userOctokit 
+ * @param {{ repoFullname: string, repoMainBranchRef: string }} repoDetails 
+ * @param {string} newBranchName 
+ * @returns New Branch/Ref details
+ * 
+ * @todo handle error for `'Reference already exists'`
+ */
+export async function createBranch(userOctokit, repoDetails, newBranchName) {
+  const { repoFullname, repoMainBranchRef } = repoDetails;
+  const { repoName, repoOwner } = getRepoParts(repoFullname);
+  const { object: { sha } } = await getBranch(userOctokit, repoFullname, repoMainBranchRef);
+
+  try {
+    const response = await userOctokit.request("POST /repos/{owner}/{repo}/git/refs", {
+      owner: repoOwner,
+      repo: repoName,
+      ref: `refs/heads/${newBranchName}`,
+      sha,
+    });
+  
+    return response.data
+  } catch (error) {
+    throw new Error("error creating a new branch", { 
+      cause: error.message 
+    })
+  }
+}
+
+/**
+ * Get a Branch/Ref details
+ * @param {import("octokit").Octokit} userOctokit 
+ * @param {string} repo 
+ * @param {string} ref 
+ * @returns Branch/Ref details
+ */
+export async function getBranch(userOctokit, repo, ref) {
+  const { repoOwner, repoName } = getRepoParts(repo);
+
+  const response = await userOctokit.request("GET /repos/{owner}/{repo}/git/ref/{ref}", {
+    owner: repoOwner,
+    repo: repoName,
+    ref: ref,
+  });
+
+  return response.data;
+}

src/lib/fork.js

@@ -1,3 +1,4 @@
+import { getBranch } from "./branch.js";
 import { getRepoParts } from "./utils/index.js";
 
 /**
@@ -90,25 +91,6 @@ async function isRepositoryForkUpdated(userOctokit, repoDetails, fork) {
   };
 }
 
-/**
- * Get a Branch/Ref details
- * @param {import("octokit").Octokit} userOctokit 
- * @param {string} repo 
- * @param {string} ref 
- * @returns Branch/Ref details
- */
-async function getBranch(userOctokit, repo, ref) {
-  const { repoOwner, repoName } = getRepoParts(repo);
-
-  const response = await userOctokit.request("GET /repos/{owner}/{repo}/git/ref/{ref}", {
-    owner: repoOwner,
-    repo: repoName,
-    ref: ref,
-  });
-
-  return response.data;
-}
-
 /**
  * Check for the presence of a specific repo in a user's fork repo list
  * @param {import("octokit").Octokit} userOctokit