Test abstraction references

[leobalter]

Raw idea:

• ECMA-402: Every build will check references in the glossary, failing if not found in ECMA-262.
• ECMA-262: Every build will import the references from glossary files we mark (checkout repo's default branch and lookup). Instead of a failure, warn if new changes affect references.

This avoids a vicious cycle if we actually need to change abstraction names.

Originally posted by @leobalter in #387 (comment)

[sffc]

To be clear, this bug is suggesting that we add/improve continuous integration testing on the repo?

[sffc]

CC @ljharb @gibson042