parser: add or when to case_statement

Fixes #68

grammar.js

@@ -895,15 +895,14 @@ module.exports = grammar({
     _case_terminator: ($) =>
       choice($._block_terminator, seq(kw("END"), kw("CASE"), $._terminator)),
 
-    _case_branch_body: ($) => choice($.do_block, $._terminated_statement),
+    _case_branch_body: ($) =>
+      choice($.do_block, field("statement", $._terminated_statement)),
+
+    case_conditon: ($) =>
+      seq(optional(seq(kw("OR"), kw("WHEN"))), $._expression),
 
     case_when_branch: ($) =>
-      seq(
-        kw("WHEN"),
-        field("condition", $._expression),
-        kw("THEN"),
-        $._case_branch_body
-      ),
+      seq(kw("WHEN"), repeat($.case_conditon), kw("THEN"), $._case_branch_body),
     case_otherwise_branch: ($) => seq(kw("OTHERWISE"), $._case_branch_body),
 
     case_body: ($) =>

test/corpus/basic.txt

@@ -1595,6 +1595,7 @@ CASE test:
     x = 5.
     y = 20.
   END.
+  WHEN "yyy" OR WHEN "zxc" THEN y = 3.
   WHEN "def" THEN y = 10.
   OTHERWISE c = 8.
 END CASE.
@@ -1606,7 +1607,8 @@ END CASE.
     (identifier)
     (body
       (case_when_branch
-        (double_quoted_string)
+        (case_conditon
+          (double_quoted_string))
         (do_block
           (body
             (variable_assignment
@@ -1618,7 +1620,19 @@ END CASE.
                 (identifier)
                 (number_literal))))))
       (case_when_branch
-        (double_quoted_string)
+        (case_conditon
+          (logical_expression
+            (double_quoted_string)
+            (identifier)))
+        (case_conditon
+          (double_quoted_string))
+        (variable_assignment
+          (assignment
+            (identifier)
+            (number_literal))))
+      (case_when_branch
+        (case_conditon
+          (double_quoted_string))
         (variable_assignment
           (assignment
             (identifier)