handle stab clauses without right-hand-sides

Currently a stab clause without a right-hand-side is parsed as an error:

```elixir
Enum.map(xs, fn x ->
end)
```

And the `end` token ends up not being highlighted as a keyword. The
compiler gives a warning about this syntax but it comes up pretty
often when editing (writing a `case` block for example).

Implementation-wise, this might be a bug in tree-sitter? `prec.right`
seems to fight with error recovery when the rightmost token(s) are
`optional`.

```elixir
fn ->
end
```

gets parsed as

```scm
(anonymous_function
  (stab_clause
    right: (body (identifier)))
  (MISSING "end"))
```

although the `optional` should allow this case. I've seen this in
other grammars and it seems like the way around it is to replace
the `prec.right` with a conflict.

Author: the-mikedavis

grammar.js

@@ -159,6 +159,16 @@ module.exports = grammar({
     //   * stab arguments item in `arg1, left when right ->`
     [$.binary_operator, $._stab_clause_arguments_without_parentheses],
 
+    // Given `( -> • \n`, the newline could be either:
+    //   * stab clause without a body
+    //   * stab clause with a body
+    [$._stab_clause_without_body, $._stab_clause_with_body],
+
+    // Given `( -> • /`, `/` token could be either:
+    //   * stab clause with a body
+    //   * -> as an operator followed by `/`
+    [$._stab_clause_with_body, $.operator_identifier],
+
     // Given `((arg1, arg2 • ,`, `arg3` expression can be either:
     //   * stab parenthesised arguments item in `((arg1, arg2, arg3) ->)`
     //   * stab non-parenthesised arguments item in `((arg1, arg2, arg3 ->))`
@@ -729,13 +739,20 @@ module.exports = grammar({
       ),
 
     stab_clause: ($) =>
-      // Right precedence, because we want to consume body if any
-      prec.right(
-        seq(
-          optional(field("left", $._stab_clause_left)),
-          field("operator", "->"),
-          optional(field("right", $.body))
-        )
+      choice($._stab_clause_with_body, $._stab_clause_without_body),
+
+    _stab_clause_with_body: ($) =>
+      seq(
+        optional(field("left", $._stab_clause_left)),
+        field("operator", "->"),
+        field("right", $.body)
+      ),
+
+    _stab_clause_without_body: ($) =>
+      seq(
+        optional(field("left", $._stab_clause_left)),
+        field("operator", "->"),
+        optional($._terminator)
       ),
 
     _stab_clause_left: ($) =>

test/corpus/do_end.txt

@@ -683,6 +683,28 @@ end
         (body
           (integer))))))
 
+=====================================
+stab clause / edge cases / empty right-hand-sides
+=====================================
+
+fun do
+  x ->
+  y ->
+end
+
+---
+
+(source
+  (call
+    (identifier)
+    (do_block
+      (stab_clause
+        (arguments
+          (identifier)))
+      (stab_clause
+        (arguments
+          (identifier))))))
+
 =====================================
 pattern matching
 =====================================

test/highlight/anonymous.ex

@@ -41,6 +41,12 @@ fn x, y, z ->
   end
 end
 
+fn ->
+# <- keyword
+#   ^ operator
+end
+# <- keyword
+
 &Set.put(&1, &2)
 # <- operator
 # ^ module