Set up build for Web extension

[yhatt]

This is the first try to make a build for VS Code Web extension.

Closes #281.

Confirmed

• Action button and commands
• IntelliSense for Marp Markdown
• Marp preview
  • While debugging, it looks like that VS Code is ignored extendMarkdownIt contribution point.
• Custom themes
• Export
  • We have no plans to support an export command on the Web for now. Export command is same meaning as the frontend of Marp CLI, and CLI does not assume working on the Web.

ToDo

• Set up build system for Web extension
  • webpack
    • We figured rollup does not suit for Web bundle that requires complex settings for Node polyfills.
  • Isomorphic modules
    • Don't use Node.js modules as possible (remaining: path, os -> polyfilled)
      • fs -> workspace.fs with Uri.file()
    • Replace node-fetch to fetch-ponyfill (It still will use node-fetch on the Node.js environment)