v4 syntax TODO

Here is the current stack of diffs for v4 features.

This task list tracks the remaining items that make sense to do in a breaking release.

• Support for reason version tagging at start of file [@reason.version 3.7];
• String template support using backticks like ES6, and polymorphic variants now use tag syntax #LikeThis.
• String template literals normalize leading whitespace unlike ES6.
• Type parameters use angle brackets type doubleList<'t> = list<('t, 't)>.
• Automatically update entire reason file to v4 editor if detecting use of angle brackets for type parameters.
• Fix parsing precedence of string concatenation ++ and various operators to be left biased in the AST so that x ++ y ++ z is (x++y) ++ z.
• Figure out how to interpolate printing of types in string literals. Should we use the same construct as printf for this?

        let x = `
          By default a string is expected here ${someVariableThatIsAString}
          But what about an integer %d{int here}
          And what about a custom datatype %%{myCustomDataType, printer}
        `

• Decide identifier renaming (or not) scheme once and for all to avoid all identifier conflicts/ambiguity with ocaml identifiers.
• { } should not be an empty object and not be the empty object type. Should possibly be unit in expression and something else in types. Empty objects are seldom used.

Start with the linked pull-request commit stack for the v4 features and compare it with each unchecked item in this issue. Determine the affected parser, printer, and identifier-handling entry points before taking on one remaining item; done means the selected syntax behavior is decided and implemented with appropriate coverage.