[Feature-Request]: Add default or main Command to Switch Back to Main/Master Branch

### Introduction

Adding a new command, `default` or `main`, would be beneficial to the `gitnow` tool that allows users to quickly switch back to the **main** or **master** branch of their repository. This command would streamline the workflow for users who frequently switch between feature branches and the main branch.

### Suggested Implementation
The new command should:
1. Detect the default branch name (either `main` or `master`).
2. Switch to the detected default branch.

### Example Usage
**Command:**
```sh
default
```

or:

```sh
main
```

**Instead of:**
```sh
git checkout main
# or
git checkout master
```

This feature would enhance the usability of `gitnow` by providing a quick and intuitive way to switch back to the main branch, aligning with common git workflows.