fix: #4886

Overview

This PR resolves issue #4886 where _downloadFont in the LocalFontProcessor was hardcoded to use the default ofetch instead of adding the fetch option.
This prevented font downloading when using proxy configurations.

Scope of Work

• Added fetch property to LocalFontProcessorOptions interface
• Now uses options?.fetch ?? fetch to suppor fetch implementations

Example Usage

const processor = createLocalFontProcessor({
  fetch: (url: string) => 
    axios.get(url, { httpsAgent: new ProxyAgent() })
      .then(response => ({
        arrayBuffer: () => response.data
      })),
  // ... other options
})

This is my first PR for Unocss. If you have any feedback or suggestions, please let me know.