[flow][ast] add one-sided (implies) type guard variant

Summary:
We will be adding a qualifier to type guard definition to express the notion of a "one-sided" refinement (see [relevant TS issue](microsoft/TypeScript#15048)). The new syntax will be:
```
function foo(x: T): implies x is R { ... }
```
A one-sided type guard will only refine the then-branch of a conditional.

We are adding this feature to unblock stricter [consistency checking](https://flow.org/en/docs/types/type-guards/#predicate-type-is-consistent-with-refined-type) for the else case of default type guards (two-sided). One-sided type guards continue to have the existing consistency checking. See this exchange for more context: https://twitter.com/danvdk/status/1765745099554668830.

This diff adds AST support.

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D56506513

fbshipit-source-id: fb2adb11ca184bbf983973fa4d7369c5aa5f7cdb

src/parser/estree_translator.ml

@@ -1643,6 +1643,7 @@ with type t = Impl.t = struct
         match kind with
         | Default -> null
         | Asserts -> string "asserts"
+        | Implies -> string "implies"
       in
       node
         ?comments:(format_internal_comments comments)

src/parser/flow_ast.ml

@@ -670,6 +670,7 @@ and Type : sig
     and kind =
       | Default
       | Asserts
+      | Implies
 
     and ('M, 'T) t' = {
       kind: kind;

src/parser_utils/output/js_layout_generator.ml

@@ -3571,6 +3571,7 @@ and type_guard ~opts ~needs_parens guard =
     match kind with
     | Default -> []
     | Asserts -> [Atom "asserts"]
+    | Implies -> [Atom "implies"]
   in
   let id_part = identifier x in
   let type_part =

src/typing/errors/flow_intermediate_error.ml

@@ -3828,6 +3828,7 @@ let to_printable_error :
         match kind with
         | Flow_ast.Type.TypeGuard.Default -> "This kind of type guard is"
         | Flow_ast.Type.TypeGuard.Asserts -> "Type guard assertions are"
+        | Flow_ast.Type.TypeGuard.Implies -> "One-sided type guards are"
       in
       [text (kind_str ^ " not yet supported.")]
     | MessageUnsupportedSyntax (UnsupportedInternalSlot { name; static = false }) ->