Migrate to ecmarkup

[Huxpro]

Summary

This is the first step to revamp this repo: ecmarkup is TC39's official tool for specifying syntax and semantics for ECMAScript. I proposed that we should adopt it because it has several favorabilities than the current approach (README.md + a gh-pages website living in the websitescript branch):

1. The resultant index.html has a similar looks and feels with the official ECMA-262 spec.
2. It uses Grammarkdown which is not only convenient to write but more importantly, can enforce the grammar to be more formal.
3. It provides links to ECMA-262 out of the box (close #112). Furthurmore, I can use <ins> to make "extension" points explicit.
4. It can became the single source of truth and prevent from syncing issue between the README and website (close #128)

One possible concern is that it makes this spec a bit like a ECMAScript proposal, but I think we mediate it by opting-out from the "stage-N proposal" heading, and by explicitly calling out our intention in the "Introduction" section.

In addition,

• Parser and Transpiler implementation section is not took over.
• Facebook, Inc. is updated to Meta Platform, Inc.

Once this is merged, we can switch to main to host Github Pages.

Test Plan

open index.html

[Huxpro]

Hey @Jack-Works this has been merged. Would you be interested in submitting a PR for the Early Errors? You may use the "Editorial" tag for that.