Use src instead of lib for directory structure

[jamos-tay]

Taken from #467 (comment)

As suggested with src/plugins/render-tags.js, let's start using src/ instead of lib/.

• src/ is pretty common.
• libs/ implies libraries' code, rather than this library's code. It's more nested but clearer.

Current directory structure:

lib/
  asset/
  markbind/
    lib/
      markdown-it/
      parser.js
      utils.js
    package-lock.json
    package.json
  template/
  util/
  Page.js
  Site.js
package-lock.json
package.json

Proposed directory structure:

src/
  asset/
  lib/
    markbind/
      src/
        lib/
          markdown-it/
        parser.js
        utils.js
      package-lock.json
      package.json
  template/
  util/
  Page.js
  Site.js
package-lock.json
package.json