Add main entry point and type definitions

[yhatt]

Update package.json to add main entry point and type definitions.

Basically Marp CLI has not provided internal API for Node because we want to focus a behavior in the scope of CLI/CUI. However, an exported function from entry point is useful as a minimum interface to use CLI via Node.

const marp = require('@marp-team/marp-cli')

(async () => {
  const exitCode = await marp(['marp.md', '-o', 'marp.html'])
  if (exitCode === 0) {
    // succeeded
  } else {
    // error
  }
})()

This function may be used in Marp for VS Code: marp-team/marp-vscode#4