Add :> (unchecked cast) operator to tree-sitter grammar

Summary:
The eqwalizer unchecked cast macro expands to (Expr :> Type), but tree-sitter
did not recognize :> as a valid token, causing parse errors. This extends
ann_var to accept choice('::', ':>') so both checked and unchecked casts parse
correctly.

Reviewed By: alanz

Differential Revision: D100770553

fbshipit-source-id: a14f816f75128377a34c7f0b8df101e17d78a75e

Author: michalmuskala

grammar.js

@@ -538,7 +538,7 @@ module.exports = grammar({
 
         ann_type: $ => prec(PREC.DCOLON, seq(field("var", $.ann_var), field("ty", $._expr))),
 
-        ann_var: $ => prec(PREC.DCOLON, seq(field("var", $.var), '::')),
+        ann_var: $ => prec(PREC.DCOLON, seq(field("var", $.var), choice('::', ':>'))),
 
         // Both union type and list tail
         pipe: $ => prec.right(PREC.PIPE, seq(field("lhs", $._expr), '|', field("rhs", $._expr))),

queries/highlights.scm

@@ -189,6 +189,7 @@
  "<-"
  "#"
  "::"
+ ":>"
  "|"
  ":"
  "="