Reason V4 [Stacked Diff 3/n #2614] [Parse Hashtags for polymorphic variants]

Summary:

Implements parsing for "hashtags" polymorphic variant constructors.
Since Reason Syntax still supports object syntax, we needed to rearrange
some syntactic real estate to make this work.

```reason
let red = #FF000;

let isRed = color => switch(color) {
  | #FF0000 => true
  | _ => false
};

let callAMethod = someObject::methodName(isRed, "testing red");

let templateLiteral = `
  String template literals are still using backticks.
  String template literals are still using backticks.
`;

```

Test Plan:

Reviewers:

CC:

Author: jordwalke

esy.json

@@ -85,6 +85,6 @@
   },
   "scripts": {
     "test": "esy x make test-once-installed",
-    "doc": "esy dune build @doc"
+    "doc": "esy build dune build @doc"
   }
 }

formatTest/typeCheckedTests/expected_output/arityConversion.re

@@ -1,4 +1,5 @@
 [@reason.version 3.7];
+
 Some((1, 2, 3));
 
 type bcd =

formatTest/typeCheckedTests/expected_output/attributes.re

@@ -7,9 +7,9 @@
  * This has a nice side effect when printing the terms:
  * If a node has attributes attached to it,
  */;
-[@reason.version 3.7];
 
 /**Floating comment text should be removed*/;
+[@reason.version 3.7];
 
 /**
  * Core language features:

formatTest/typeCheckedTests/expected_output/autoUpgradeAngleBrackets.re

@@ -0,0 +1,14 @@
+/**
+ * Even if you have an explicit v3.6 marker.
+ * This whole file wil be auto-upaded to 3.8 becase something uses
+ * angle brackets.
+ */;
+[@reason.version 3.8];
+let watchThisIsOldStyle: list<int> = [1, 2];
+
+let watchThisIsOldStylePoly = #hello;
+
+/**
+ * This will cause the whole file to be promoted.
+ */
+let x: list<int> = [1, 3];

formatTest/typeCheckedTests/expected_output/autoUpgradeAngleBracketsNoVersionAttr.re

@@ -0,0 +1,13 @@
+[@reason.version 3.8];
+/**
+ * Test auto-promotion based on feature inference even if no version
+ * tag. By default you're using the old 3.7.
+ */
+let watchThisIsOldStyle: list<int> = [1, 2];
+
+let watchThisIsOldStylePoly = #hello;
+
+/**
+ * This will cause the whole file to be promoted.
+ */
+let x: list<int> = [1, 3];

formatTest/typeCheckedTests/expected_output/autoUpgradeDoNotAutoUpgrade.re

@@ -0,0 +1,5 @@
+[@reason.version 3.7];
+/**
+ * This should just print a 3.7 version attr at the top.
+ */
+let watchThisIsOldStyle: list(int) = [1, 2];

formatTest/typeCheckedTests/expected_output/comments.re

@@ -1,10 +1,11 @@
 /* **** comment */
 /*** comment */
-/** docstring */;
+/*** docstring */;
+
 [@reason.version 3.7];
 
 /* comment */
-/** docstring */;
+/*** docstring */;
 /*** comment */
 /**** comment */
 /***** comment */

formatTest/typeCheckedTests/expected_output/comments.rei

@@ -1,15 +1,16 @@
 /* **** comment */
 /*** comment */
 /*** docstring */
+
+[@reason.version 3.7];
+
 /* comment */
 /*** docstring */
 /*** comment */
 /**** comment */
 /***** comment */
 
 /** */;
-[@reason.version 3.7];
-
 /*** */
 /**** */
 

formatTest/typeCheckedTests/expected_output/comments.rei.4.07.0

@@ -1,6 +1,7 @@
 /* **** comment */
 /*** comment */
 /** docstring */;
+
 [@reason.version 3.7];
 
 /* comment */

formatTest/typeCheckedTests/expected_output/comments.rei.4.07.1

@@ -1,6 +1,7 @@
 /* **** comment */
 /*** comment */
 /** docstring */;
+
 [@reason.version 3.7];
 
 /* comment */