This is the last step to make https://github.com/babel/babel/pull/9364 usable in Babel. I'm sorry for opening this PR so late, but I hope to get it in v7.4.
In this PR I added a new option to `@babel/template`, `syntacticPlaceholders: ?boolean`, which toggles between `%%foo%%` placeholders (when `true`) and `FOO` placeholders. If it isn't specified, Babel tries to be "smart" to avoid breaking backward compat: if `%%foo%%` is used `syntacticPlaceholders` defaults to `true`, otherwise to `false`.
0e58e252913efe84eba926cc9c9c19fb18d5c620 commit shows how some templates we used could be simplified by using this new placeholders syntax (we can't actually do it yet because we are importing `template` from `@babel/core` which could be an older version).
NOTE: Since I wanted to keep this PR as small as possible to make it easier to review, I didn't migrate `template.ast` to internally use the new syntax. It is an implementation detail, so it will be possible to change it in a patch release.
