WIP: refactor: migrate to ESM

[yoshinorin]

WIP

• Suspended by chore: require Node.js 14+ & CI use Node.js 16 #129
• Fix test failure
• Fix lint
• nyc does not support ESM (use c8 instead)
• Node 14 has error

[stevenjoezhang]

We could change typescript settings to generate ESM now.

[SukkaW]

Although some of Node.js community members stated that the cost of transition is minimum, but unfortunately this isn't true for Hexo. We can (and we should) experimenting with ESM, but I will block this PR from being merged, since ESM is still not ready yet. Here are why:

• Import Map does not work as expected for most of Node.js versions.
• Inconsistent default import/export behavior across runtime/bundler.
• We can't replace dynamic require with dynamic import. Hexo uses dynamic require for lazy-loading modules in order to improve cold boot performance and reduce the memory usage. While dynamic require is synchronous, dynamic import is asynchronous and we will have to make huge change to our existing code bases (mostly plugin loading, themes' scripts loading, extensions loading, etc.), and eventually we will have to introduce breaking changes that will break most of existing themes and plugins.

[D-Sketon]

I think this PR can be closed