[Bug] Missing Yarn Dependency Check During Installation

**Describe the bug**
When installing the Markdown Preview plugin, it does not check for the presence of Yarn. As a result, if Yarn is not installed, the plugin fails to work, but this failure is not immediately apparent.

**To Reproduce**
Steps to reproduce the behavior:

1. Install the Markdown Preview plugin without Yarn installed.
2. Attempt to use the plugin.
3. Notice that the plugin does not work due to the missing Yarn dependency.

**Expected behavior**
The installation process should check for the presence of Yarn. If Yarn is not installed, the user should be notified with a clear message indicating that Yarn is required for the plugin to function properly.

**Screenshots**
N/A (The issue is about the installation process and not a runtime error that can be captured in a screenshot).

**Desktop (please complete the following information):**
- OS: macOS Ventura 13.6.7 arm64
- Host: MacBook Air (M1, 2020)
- Kernel: Darwin 22.6.0
- Shell: bash 3.2.57

Log:

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