* Permit %%placeholder%% in left-hand-side of a let declaration
* Test that "let" followed by modulo is still treated as an identifier
* More tests for edge-case handling of "let" with placeholders enabled
* Allow templates to parse v8intrinsics
The `v8intrinsic` and `placeholders` parser plugins conflict, so enabling `placeholders` unconditionally was causing errors for V8's internal codemods. This allows them to set `syntacticPlaceholders = false` (so they'll use the legacy identifier format) and enable `v8intrinsic` by itself.
* Fix linter
* Linter
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.
