feat(tolk): support ?? operator from Tolk 1.3 (#256)

Fixes #255

Author: i582

server/src/e2e/tolk/testcases/types/null-coalescing.test

@@ -0,0 +1,71 @@
+========================================================================
+Null coalescing operator ?? — Basic
+========================================================================
+fun main() {
+    val a: int?;
+    val b: int;
+    val c = a ?? b;
+    //! ^ int
+}
+------------------------------------------------------------------------
+ok
+
+========================================================================
+Null coalescing operator ?? — Left is Null
+========================================================================
+fun main() {
+    val a: null;
+    val b: int;
+    val c = a ?? b;
+    //! ^ int
+}
+------------------------------------------------------------------------
+ok
+
+========================================================================
+Null coalescing operator ?? — Left is not Nullable
+========================================================================
+fun main() {
+    val a: int;
+    val b: bool;
+    val c = a ?? b;
+    //! ^ int | bool
+}
+------------------------------------------------------------------------
+ok
+
+========================================================================
+Null coalescing operator ?? — Right side type
+========================================================================
+fun main() {
+    val a: int?;
+    val b: bool;
+    val c = a ?? b;
+    //! ^ int | bool
+}
+------------------------------------------------------------------------
+ok
+
+========================================================================
+Null coalescing operator ?? — Smart cast in RHS
+========================================================================
+fun main() {
+    val a: int?;
+    val c =
+    //! ^ int?
+        a ?? a;
+    //!      ^ null
+}
+------------------------------------------------------------------------
+ok
+
+========================================================================
+Null coalescing operator ?? — Incomplete
+========================================================================
+fun main() {
+    val a: int?;
+    val c = a ??;
+    //! ^ int?
+}
+------------------------------------------------------------------------
+ok

server/src/languages/tolk/tree-sitter-tolk/grammar.js

@@ -443,6 +443,7 @@ const TOLK_GRAMMAR = {
 
     binary_operator: $ =>
         choice(
+            prec.right(10, seq($._expression, field("operator_name", "??"), $._expression)),
             prec.left(
                 13,
                 seq($._expression, field("operator_name", choice("&&", "||")), $._expression),