Extracts the parser to `@handlebars/parser`, where it can be shared
between different implementations. This means that e.g. Glimmer/Ember
will be able to iterate on new features without forcing Handlebars to
adopt them immediately, and vice versa. All implementors will be able to
absorb changes as it makes sense for them.
fixes#1548
- add a guard to show readable syntax error for if / unless helper
- prevents against 3 different errors that can be generated by different systax errors
Jakob Linskeseder
Chris Garrett
Nils Knappmeier
Hannah Wolfe
machty
kpdecker