Check for mkcert in PATH if mkcertPath not given, before attempting a download

[FrostKiwi]

Big fan of this plugin, a life saver, because certain browser features cannot be used without HTTPS. As someone developing for WebXR, essential for local testing.

Description

This PR makes vite-plugin-mkcert check for mkcert in PATH, if mkcertPath is not defined, before attempting the default download.

Additional context

This is to solve two things, which I encountered in an environment with a corporate firewall. vite-plugin-mkcert doesn't seem to work with with our proxy and fails the download with ERCONNRESET. Downloading manually and specifying mkcertPath works, but leads to a code portability problem if the downloaded mkcert is not part of the vite-projects's repo and an OS portability issue, if you check in the windows' mkcert, but another dev uses linux and vice-versa. Installing mkcert to PATH and relying on vite-plugin-mkcert to check PATH was the cleanest solution for me.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other